So, I was going to edit world 8 opening w8.nsbmd, but it says that it's already openned. So if someone can tell me what happens, that will be amazing (I use a U Psyfer rom). I also used the "already opened" fix, but when tried to subtitute, it appears the "already oppened" message. I'm using NSMBe 379.

What? You don't edit World Maps in NSMBe.

The only thing you would be able to edit with NSMBe are textures.

Use Goombatlas to create a new world map or MKDSCM to replace the model of an existing one.
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"I strikingly thought you would know how but it clearly seems you have no knowledge of hacking" - MiiU 2017

TruelyJohn64, I mean the outside of the world, the textures.

Then just use MKDSCM or Every File Editor, it's a lot more reliable for editing textures of models than NSMBe.

Also you don't have to quote and address me at the same time just as a FYI.
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"I strikingly thought you would know how but it clearly seems you have no knowledge of hacking" - MiiU 2017

Thanks for the 2 things!