Download .NET 9 for Visual Studio 2022 from the official website:
https://dotnet.microsoft.com/en-us/download/dotnet

 
Click on: Windows x64

After installation, let's do a check:
After installation, run the comad line and enter the command dotnet --info

On the screen we will see:
.NET installed:
9.0.306 [C:\Program Files\dotnet\sdk]
 

In Visual Studio 2022 right-click on the project → tab Application → Target Framework [Choosing the new version 9.0]

 
When compiling a project, error:
Error
NETSDK1 The current .NET SDK does not support targeting .NET 9.0. Either target .NET 7.0 or lower, or use a version of the .NET SDK that supports .NET 9.0.

 
Decision!
I have Visual Studio 2022 (version 17.6.5).
In order to compile a project with NET 9, you need to update Visual Studio, i.e. my version is old (17.6.5)

Updating Visual Studio 2022
At the top of the menu, click: Help → Check for Updates

 
Press Update

 
Result!
Everything compiles and NET 9 works well.