How do I update minSdkVersion and targetSdkVersion in the new version of flutter 2.8.0

You can simply change these in android/local.properties file.

define or change them inside android/local.properties:

org.gradle.jvmargs=-Xmx1536M
android.useAndroidX=true
android.enableJetifier=true

// add these lines
flutter.minSdkVersion=21
flutter.targetSdkVersion=29

flutter 2.9.2 in [prj]/android/app/src/build.gradle change :

minSdkVersion flutter.minSdkVersion

to (for example) :

minSdkVersion localProperties.getProperty('flutter.minSdkVersion')

and in [prj]/android/local.properties add/change:

flutter.minSdkVersion=XX

Replace

minSdkVersion flutter.minSdkVersion

targetSdkVersion flutter.targetSdkVersion

With this

minSdkVersion 21

targetSdkVersion 31

versionCode flutterVersionCode.toInteger()

versionName flutterVersionName