Tweezers, hair dryer, and lots of patience!
The way I've taken a vinyl dealer decal off a car once was to heat the decal(s) up with a hair dryer and CAREFULLY peel it up using the tweezers while continuing to heat it. Then I used some bug and tar remover to remove the last little bits of adhesive residue.
The adhesive will eventually soften and let go when it gets hot enough. Work slowly, carefully, and patiently and the decals will come off cleanly and easily. Then apply some bug and tar remover to clean up any remaining residue.
Use a hair dryer, not a heat gun! A hair dryer should get everything hot enough for the adhesive to soften and let go but not so hot that the paint/clearcoat gets damaged. The risk with a heat gun is that it can get things too hot and start to burn stuff.
The lettering on the back of my Golf is white reflective vinyl, put on by a local sign shop. I hope I don't have to take any of it off anytime soon.
Good luck.
~ n1das