Hot air from hairdryer to loosen, lift up edge of decal with fingernail or you can scrub (gently) with a wooden tongue depressor or wood kitchen spoon under the hot air stream. This won't hurt the paint. Remove residue with Goo-gone or any mild oil-based solvent, then finish with mild detergent and water to remove the oil.
I wouldn't recommend anything higher temp than a normal consumer hairdryer for this.