If you want to change the texture that is in a custom level to something else you need to open it in the editor, change the style to custom, and then press shift/T and type the name of the folder that contains the texture you want. Then click save. If you want to still keep the original one too then change the name of the level before you save.
You can do exactly the same for backgrounds except in that case you press shift/B before typing in the name of the folder, and change the bkg to custom.
If you're making your own level do the same thing.
PS. Oh, I forgot to say, if the level is in a folder you'll need to take it out and place it directly in the custom levels folder to be able to open it in the editor. Once you've done you can put it back in the folder it came from.
