A hair dryer works best to loosen the adhesive used to keep sticker/decals in place. Go slow and be aware that too much heat could damage paint. Here, as we use it on glass, that is not a problem.
Heat an edge of the sticker/decal and try to peel it away. As it loosens, pull more of the sticker away from the surface until it’s free.
Here, removing the sticker from glass, we’ve used a razor blade to gently scrap away excess adhesive. DO NOT try this when the sticker is applied to a painted surface! When the surface is painted, use an adhesive remover, such as Goof Off.
After the excess adhesive has been removed: For cleanup on glass, use a good quality glass cleaner. Here we’re using Rain-X. It’s an automotive glass cleaner that doesn’t contain ammonia, which can be harmful to your car’s glass. For cleanup on painted surfaces, use a good quality liquid car wash, such as Meguair’s Deep Crystal Car Wash.