How to make a mesh collider out of a prefab?

Hey, guys. So, I downloaded an environment for my game online, but, had to delete some stuff to make it look the way I want it to. But now, when I use the environment’s Mesh Collider, the things I deleted still appear in the collider. I made a Prefab for the environment, and now want to use the prefab to make an updated Mesh Collider for the environment. Does anybody know how to do this using the Unity Editor and C# code? If so, I’d greatly appreciate if you could show me how. Thanks in advance!

Do you mean by you keep falling through(No solid ground) or it keeps going to its original state?

If you keep falling:
1: Make sure it has a mesh collider attached in inspector
2: If you do and still have the issue then add a box collider and measure it out.

If it keeps going to its original state(I have not had this before so bear with me):
1: Close Unity and reopen it
2: If that does not work check to make sure the prefab itself does not contain the stuff you do not want
3: If all else fails then contact the original creator to have them re-do it or if it is a .blend get blender and make your changes.

You should be able to just remove the MeshCollider component from the environment and add a new mesh collider to it, Unity should create a new one based on its layout.