how can I change the .NET to .NET Framework 4.7.2 i am really struggeling since 2 days now. I am doing a WinFormApp and can only use .NET 5 or .NET Core 3.1 but i need .NET Framework 4.7.2 for another Framework. Already installed it through [Microsoft] but it dont appear there to select.
You don't need to "restart a project" to target .NET Framework. You just need to use the appropriate TargetFramework element in the csproj file.
Here's an example for a Windows Forms application targeting .NET Framework 4.7.2:
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<OutputType>WinExe</OutputType>
<TargetFramework>net472</TargetFramework>
<UseWindowsForms>true</UseWindowsForms>
</PropertyGroup>
</Project>
If you create a .NET Core 3.1 or .NET 5 project, you can just edit the .csproj file to specify .NET Framework as the target framework - no need to start from scratch.
Well, as your main application is .NET Core, it's running on a core CLR. It cannot be changed to a non-core CLR project. You would need to restart a project if there is no way of achieving the goal you need.
If you also need the .NET Core application for whatever reason, I would recommend using an API call to communicate between the two applications or another method of communication which isn't with the project itself.