Like James said use a dull scraper such as an old plastic ice scraper or make a scraper using 1/4" plexiglass. Stay away from a metal scraper. You will then end up with old adhesive on the fiberglass, to clean this use Goo-Gone, Dissolve-it or other adhesive cleaner. After all the adhesive has been removed and the area has been cleaned wipe very lightly with acetone to help remove the wax and gloss from under the old decal then compound or polish and buff the area under the decal to try to blend it in with the rest of the finish. Note: Acetone will disolve gelcoat use with care.