We use an electric heat gun at our custom sign shop. (most hardware stores, wal mart). Depending on how large the decal is, briefly heat the entire surface or the area you want to start. Then peel the decal and continue heating as you go. There will likely be a film of the adhesive left. Use "goo gone" (hardware or auto parts store)
on a rag and wipe the adhesive off. You wont need any type of scraper, etc., as that may scratch the surface.
Before applying the new decal, wipe the area with alcohol. To make the application smooth, we use a product called "splash" application fluid. (
www.wensco.com) Spray the surface, apply the decal and squeege out the air bubbles. Sounds compliated but the entire process takes about 20minutes for most larger decals up to 48 inches long. Hope this helps.