Although vinyl decals are generally installed to be permanent, they can easily be removed. Simply heat the decal with a heat gun or hair dryer (hair dryer will take longer). Then using your fingernail, lift the edge of the decal and work across the surface of the decal. Caution! Overheating the decal will result in more difficult removal because it can actually harden to the vehicle surface.
For glass windows, many people use the same razor that removes inspection stickers to remove unwanted vinyl decals. Do not use razors on the vehicles body paint! Once the vinyl decal is removed, use PREPSOL or thinners to remove any glue residue. That is it!