According to the Documentation this is way

1 Open your Project in Unity, and go to Edit > Rendering > Materials.

2 According to your needs, select either Convert All Built-in Project Materials to URP or Convert Selected Built-in Materials to URP.

However installing the URP in a sample project I am not seeing the Rendering menu. Possibly a bug in the beta or URP 12.0 package. It was working in previous alpha builds

https://docs.unity3d.com/Packages/com.unity.render-pipelines.universal@12.0/manual/upgrading-your-shaders.html