Change web app .NET framework version with ARM template

The answer for me was setting the property netFrameworkVersion to v6.0. No other change was necessary.

This is the partial ARM template, the section changed:

{
  "name": "[parameters('webSiteName')]",
  "type": "Microsoft.Web/sites",
  "apiVersion": "2019-08-01",
  "location": "[parameters('location')]",
  "tags": {
    "[concat('hidden-related:', resourceId('Microsoft.Web/serverfarms', variables('hostingPlanName')))]": "empty"
  },
  "dependsOn": [
    "[variables('hostingPlanName')]",
    "[variables('appInsightsName')]"
  ],
  "properties": {
    "name": "[parameters('webSiteName')]",
    "serverFarmId": "[resourceId('Microsoft.Web/serverfarms', variables('hostingPlanName'))]",
    "siteConfig": {
      "appSettings": [
        {
          "name": "APPINSIGHTS_INSTRUMENTATIONKEY",
          "value": "[reference(resourceId('Microsoft.Insights/components/', variables('appInsightsName')), '2020-02-02-preview').InstrumentationKey]"
        },
        {
          "name": "APPLICATIONINSIGHTS_CONNECTION_STRING",
          "value": "[reference(resourceId('Microsoft.Insights/components', variables('appInsightsName')), '2020-02-02-preview').ConnectionString]"
        }
      ],
      "connectionStrings": [
        {
          "name": "DefaultConnection",
          "connectionString": "[concat('Data Source=tcp:', reference(resourceId('Microsoft.Sql/servers/', variables('sqlServerName'))).fullyQualifiedDomainName, ',1433;Initial Catalog=', parameters('databaseName'), ';User Id=', parameters('databaseUserLogin'), '@', reference(resourceId('Microsoft.Sql/servers/', variables('sqlServerName'))).fullyQualifiedDomainName, ';Password=', parameters('databaseUserPassword'), ';')]",
          "type": "SQLAzure"
        }
      ],
      "ftpsState": "FtpsOnly",
      "netFrameworkVersion": "v6.0"
    },
    "httpsOnly": true
  }
}

You can find the full resource and ARM template at: https://github.com/feliperomero3/MoneySmart/blob/e2210eb6217eb6e06a299870acaf115d2e50c6a5/AzureResourceGroup/azuredeploy.json

Except for the netFrameworkVersion you need to change, also you need to add another resource to the resources in your template, just add it like below, it works fine on my side.

Note: It is for the web app whose OS is Windows.

    {
        "type": "Microsoft.Web/sites/config",
        "apiVersion": "2018-11-01",
        "name": "[concat(parameters('webapp_name'), '/metadata')]",
        "location": "Central US",
        "dependsOn": [
            "[resourceId('Microsoft.Web/sites', parameters('webapp_name'))]"
        ],
        "properties": {
            "CURRENT_STACK": "dotnet"
        }
    }

My complete sample:

{
    "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
    "contentVersion": "1.0.0.0",
    "parameters": {
        "webapp_name": {
            "defaultValue": "joyweb11",
            "type": "String"
        },
        "serviceplan_resourceid": {
            "defaultValue": "/subscriptions/xxxxx/resourceGroups/xxxx/providers/Microsoft.Web/serverfarms/joyplan",
            "type": "String"
        }
    },
    "variables": {},
    "resources": [
        {
            "type": "Microsoft.Web/sites",
            "apiVersion": "2018-11-01",
            "name": "[parameters('webapp_name')]",
            "location": "Central US",
            "tags": {},
            "kind": "app",
            "properties": {
                "enabled": true,
                "hostNameSslStates": [
                    {
                        "name": "[concat(parameters('webapp_name'), '.azurewebsites.net')]",
                        "sslState": "Disabled",
                        "hostType": "Standard"
                    },
                    {
                        "name": "[concat(parameters('webapp_name'), '.scm.azurewebsites.net')]",
                        "sslState": "Disabled",
                        "hostType": "Repository"
                    }
                ],
                "serverFarmId": "[parameters('serviceplan_resourceid')]",
                "reserved": false,
                "isXenon": false,
                "hyperV": false,
                "siteConfig": {},
                "scmSiteAlsoStopped": false,
                "clientAffinityEnabled": true,
                "clientCertEnabled": false,
                "hostNamesDisabled": false,
                "containerSize": 0,
                "dailyMemoryTimeQuota": 0,
                "httpsOnly": false,
                "redundancyMode": "None"
            }
        },
        {
            "type": "Microsoft.Web/sites/config",
            "apiVersion": "2018-11-01",
            "name": "[concat(parameters('webapp_name'), '/web')]",
            "location": "Central US",
            "dependsOn": [
                "[resourceId('Microsoft.Web/sites', parameters('webapp_name'))]"
            ],
            "tags": {
                
            },
            "properties": {
                "numberOfWorkers": 1,
                "defaultDocuments": [
                    "Default.htm",
                    "Default.html",
                    "Default.asp",
                    "index.htm",
                    "index.html",
                    "iisstart.htm",
                    "default.aspx",
                    "index.php",
                    "hostingstart.html"
                ],
                "netFrameworkVersion": "v5.0",
                "requestTracingEnabled": false,
                "remoteDebuggingEnabled": false,
                "remoteDebuggingVersion": "VS2019",
                "httpLoggingEnabled": false,
                "logsDirectorySizeLimit": 35,
                "detailedErrorLoggingEnabled": false,
                "publishingUsername": "$joyweb11",
                "azureStorageAccounts": {},
                "scmType": "None",
                "use32BitWorkerProcess": true,
                "webSocketsEnabled": false,
                "alwaysOn": true,
                "managedPipelineMode": "Integrated",
                "virtualApplications": [
                    {
                        "virtualPath": "/",
                        "physicalPath": "site\\wwwroot",
                        "preloadEnabled": true
                    }
                ],
                "loadBalancing": "LeastRequests",
                "experiments": {
                    "rampUpRules": []
                },
                "autoHealEnabled": false,
                "localMySqlEnabled": false,
                "ipSecurityRestrictions": [
                    {
                        "ipAddress": "Any",
                        "action": "Allow",
                        "priority": 1,
                        "name": "Allow all",
                        "description": "Allow all access"
                    }
                ],
                "scmIpSecurityRestrictions": [
                    {
                        "ipAddress": "Any",
                        "action": "Allow",
                        "priority": 1,
                        "name": "Allow all",
                        "description": "Allow all access"
                    }
                ],
                "scmIpSecurityRestrictionsUseMain": false,
                "http20Enabled": false,
                "minTlsVersion": "1.2",
                "ftpsState": "AllAllowed",
                "reservedInstanceCount": 0
            }
        },
        {
            "type": "Microsoft.Web/sites/hostNameBindings",
            "apiVersion": "2018-11-01",
            "name": "[concat(parameters('webapp_name'), '/', parameters('webapp_name'), '.azurewebsites.net')]",
            "location": "Central US",
            "dependsOn": [
                "[resourceId('Microsoft.Web/sites', parameters('webapp_name'))]"
            ],
            "properties": {
                "siteName": "[parameters('webapp_name')]",
                "hostNameType": "Verified"
            }
        },
        {
            "type": "Microsoft.Web/sites/config",
            "apiVersion": "2018-11-01",
            "name": "[concat(parameters('webapp_name'), '/metadata')]",
            "location": "Central US",
            "dependsOn": [
                "[resourceId('Microsoft.Web/sites', parameters('webapp_name'))]"
            ],
            "properties": {
                "CURRENT_STACK": "dotnet"
            }
        }
    ]
}