1. Go to Edit Tileset (it looks like that -> https://imgur.com/o6wZuxy)
2. Go to Graphics
3. Choose Palette 1 (expect if you want to have weird textures to edit xD)
4. Click on Export this bitmap
5. Edit your tileset with Photoshop, paint.net, GIMP...
6. Once you have finished to edit your tileset, click on Import this bitmap and recreate palette
7. Click on Save Tileset

And here you go ! You can play with your custom tileset !

You could've used screenshots as well, so anyone can understand better.

You can also use "Export all bitmaps with all palettes" and "Import all bitmaps with all palettes" options as well
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You should, too. A single palette doesn't have many colors. That said, pretty most of NSMB's tilesets don't benefit from using multiple palettes at all. They're pretty low on colors.
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Nothing to say, so jadnjkfmnjamnfjkldnajfnjkanfjdksan jsdnvj m.