In the Application tab in your screenshot. There you also need to change the TargetFramework to target Android 10.
If you still are using packages.config instead of PackageReference to manage NuGet packages. You will need to remove the NuGet packages and add them again or manually update the entries in there to grab stuff for the correct target.
So you need to update:
- TargetFramework
- AndroidManifest
- If you are using PackageReference clean bin/obj
- If you are using packages.config, remove all packages and add them back or manually update packages.config to point at correct TFM in nuget package