Warm the surface of the decal with a heat gun or hair dryer until the vinyl becomes more flexible.
Take a plastic blade tool and scrape vinyl from the surface. (Note: if the vinyl is old and brittle, it will most likely come off in small pieces. If the vinyl is not that old you may be able to pull it off in bigger sections.)
Once you have peeled all of the vinyl off, you will have to clean the adhesive off the surface with a product designed to remove adhesive without damaging the paint. ALWAYS SPOT TEST AN INCONSPICUOUS AREA BEFORE APPLYING ADHESIVE REMOVER.
A Suggested product is “General Purpose Adhesive Cleaner” by 3M Products Inc. part # 08947. This adhesive cleaner can be purchased at most hardware or paint supply stores. Clean the surface with soap and water, then check to make sure there are no remaining pieces of vinyl or adhesive stuck to it.