RTR models lettering is not done by decals, it is usually printed on with special ink/paint. There are several different methods of removing lettering, not all work on all models, even by the same manufacturer.
I've used Walthers Solvaset decal solvent on a cotton swab to remove lettering on some models. It works some times, but you need to watch carefully as you go along, as the paint underneath can also be removed. Others have used waterless hand cleaner, rubbing alcohol, commercial model paint remover, etc. There is no one solution for all models, its mostly a matter of experimenting.