How to reassign the Custom shaders for each materials to URP?

Hello Everyone .
We created a project in Unity with URP (To have a better performance in the Virtual Reality game we are doing) and added an environment to it through an unitypackage.Then we opened it and we had no problems in converting the materials and shaders the complete scene to URP, but my intention now is adding characters from another unitypackage to this URP project, but this time we have to reassign the shaders for URP manually, for it, Does anybody know how to do it?
Do you know some tutorial about this topic?
Thanks for your time.
Hopefully you can understand me since that my English is terrible