There is some good documentation here how to manually upgrade to .NET 6: https://docs.microsoft.com/en-us/aspnet/core/migration/50-to-60
But I have a large solution with many projects that references many of NuGet-packages that need to be upgraded. Is there any automatic way to do upgrade the project files?
I used to do it with the "replace-in-files option" and NuGet-Manager in Visual Studio. Worked out for me in a 181 projects solution.
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<OutputType>Exe</OutputType>
<TargetFramework>net5.0</TargetFramework>
</PropertyGroup>
</Project>
Press Shift+Ctrl+H
Open NuGet-Manager for Solution (not project itself). Select all packages and update them all (if possible, else leave out the once you know are not compatible)
You can set properties for all projects in the solution using a Directory.Build.props file, see: https://docs.microsoft.com/en-us/visualstudio/msbuild/customize-your-build?view=vs-2022#directorybuildprops-and-directorybuildtargets
In your case you'd use something like this:
<Project>
<PropertyGroup>
<TargetFramework>net6.0</TargetFramework>
</PropertyGroup>
</Project>
Place the file in the root of your solution, above all project folders.
You could try this PowerShell script that that upgrade all csproj-files from .NET 5 to .NET 6, including NuGet-packages:
https://gist.github.com/pekspro/24fca2eb1cad8c30333a910b163a3f6a
On the same location there is a script that downgrades to .NET 5. But do not trust this, make sure you have a backup if something goes wrong.