Easy and cheap, good results.
1-Hand-held hair dryer
2-Re-Solv It ($3-$4 any supermarket)
3-credit card you won't be using, or a good fingernail
4-WD-40 and a couple of shop rags.
5-Paper towels.
6-Rubbing compound.
7-1 Beer.

Apply some Re-Solv It liberally to each decal. This softens them and attacks the adhesive.
Wipe it off with a paper towel.
Heat the decal up with a hair dryer, and while it's hot, scrape with fingernail or credit card.
Once it's off, WD-40 on a shop rag and wipe off the excess adhesive.
Lightly with the rubbing compound all over the frame to eliminate fading/color difference.
Wipe off. Now you are ready for new decals and a clear coat protective finish.
Have the beer. Good job.