The decals can be removed the same way you would any other sticker.
Just heat them up a bit with a hair-dryer for example and gently peel it off. If you have any trouble getting under the edges, a credit card tends to get the job done.
To clean up the adhesive residue, use some gauze (it doesn’t leave any bits behind like a tissue would) with lighter fluid or other alcoholic chemicals such as WD-40. Dry the spot off with any kind of absorbent tissue.