Pop-Top Decal Removal:
Depending on age and level of deterioration, some decals can be removed by carefully peeling up a small portion of the edge with either a fingernail or razor blade, then pulling away from the pop-top surface slowly. Any adhesive residue left behind should be removed with a clean rag and odorless mineral spirits; mineral spirits is one of the few solvents that will not damage painted surfaces.
If the decal is badly aged, cracked, and/or sun-faded, an auto-body eraser may be needed (you can find these erasers at an automotive paint supply store). This handy tool will remove any sticker/decal and its adhesive without damaging the vehicle surface.
A note on solvents: Solvents such as paint thinner and acetone are often used mistakenly to remove old adhesive from painted surfaces. These solvents will break down paint and finishes, making them very soft, hazy—or, in some cases, completely removing paint from its substrate. Safe adhesive removers are readily available from most auto parts stores. These products should always be tested on an inconspicuous spot of the vehicle prior to widespread use.
Pop-Top Decal Application:
When applying a sticker or decal, it is imperative that the application surface be clean and dry! Mineral spirits can be used to prepare the surface, but ample time should be allowed for any residue left from the solvent to evaporate. Be aware that mineral spirits can leave a slightly slippery film; a better surface prep solvent is denatured alcohol, which is extremely gentle on painted surfaces and evaporates very quickly, leaving a clean and very dry substrate.
When applying vinyl die-cut stickers with clear overlays, make a straight and level mark on the surface to which the sticker is being applied, about 1/16" below where the sticker will be placed. You can do this with masking tape, a small pencil, or non-staining marker that will wipe away with a small amount of solvent. Using anchor points on the vinyl cut-out (such as the bottom outside corners of letters or figures), begin applying the sticker from one end. As you apply the sticker, use a squeegee (an old credit card works great) to firmly press the sticker down flat. This will also help press out any air bubbles and keep the sticker from rippling. Work slowly! Once the sticker is securely applied, gently remove the clear overlay, peeling back at 180°. If you pull the overlay off at a 90° angle, you risk peeling off the vinyl cut-outs of the decal itself. Again, work slowly! Once the clear overlay is completely removed, peel away any masking tape and/or wipe away any marks with a rag and a small amount of solvent.
If there are bubbles in the decal that simply won't push out, you can use the sharp corner of a razorblade or the point of a needle to make SMALL pinholes in the center of the bubble, then work the air toward that hole. Be very careful doing this, as you can cause ripples.
Once your sticker is stuck, so to speak, you can drive your vehicle normally. There aren't any "cure times," so get out and drive. Will the new decal make your vehicle run better? Probably not. But it sure will look nice.