+1 on the hairdryer. I had put a clear one on my tank and got too many bubbles under it. Hair dryer worked great to get the old one off.
When I put the new one on, I wiped some mineral spirits on the tank where I was gonna put it, it gave me a couple of minutes of working time to move it around to get positioned right and to work any bubbles out. The mineral spirits eventually evaporates. This is what it looked like after.
I'll go with heat also. If it is full adhesive on the back of the old one and not just adhesive strips, you may want to slide a piece of Dental Floss under the edge and work it down over the convex curvature of the tank as you apply some heat. Finish up with a light solvent that won't harm the paint/clear coat underneath to remove any residue.
I use a radiant heat gun.
Much better than a hair drier.
An oh my, some REALLY nasty fumes start venting from the tank when doing a big decal and pad strip job.
Do it outside, and not in your garage.
I started getting a wee bit woozzy doing my TechSpec installs on a cool day inside the garage with the door closed.