How to use level that is in the files of the game

im trying to edit a level that is a normal level like bomb omb battlefield. can someone help!
im using super mario 64 rom manager

I also am trying to do that too. It's way too hard to do something that seems so simple. If we could just edit preexisting areas and export stage models from the game with the textures it would be so easy. All I wanna do is add a little bit of geometry to the castle courtyard, but honestly it seems impossible.

"can someone help!"

ps. please

To edit stage texture and not the model use quad64's texture option
To make custom models use Rom manager and Blender

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Sadly this is impossible. I asked for pilzinsel64 to implement this option and he said he will never implement it. But you can download models from internet and reload them in sm64 rom manager.