Hi and Welcome,
Hair Dryer / Heat gun, good place to start, try and get a hard plastic putty knife, warm up the area and try to remove...
Next one is a bit risky, but with lots of patience and care, you can do it... Use a brand new single edge razor blade, turned almost flat to the surface, between the surface and the decal, remove most of the decal and adhesive , then use some adhesive remover, or laquer thinner, or acetone, or even some mineral spirits , to remove the rest of the adhesive...
One tip is to dull the corners of the razor a little, might keep it from scratching...
ALWAYS test whatever chemicals you are going to use in an inconspicuos place to make sure it isn't goin to eat the finish off...
Then, finally, this is what I use, maybe you can rent a similar tool...
http://www.dynabrade.com/dyn10/prod...3JTgw&epf=JUEzJUJDJTg3&market=&vacuum=&search=
These work wonders, and if the surface is in good condition, won't hurt anything and leave it ready for a final wipe down and application of your new decals...
As a last resort, you can try some kind of chemical stripping agent, but again, TEST first...
Good Luck,
GT1M