Method #1
Heat the bumper sticker with a hair dryer or heat gun until the glue underneath softens and then try peeling it off. If you do this in the summer, let the vehicle sit in the hot sun for about half an hour first, speeds up the process. If you can’t park near an outlet to use a hair dryer and don’t have a heat gun, try pouring boiling water over the sticker to get that glue underneath melted and loosened up enough to pull off the sticker.
Method #2
Saturate the bumper sticker really well with WD40, or lighter fluid, or white vinegar and allow to sit for a few minutes then try to remove*
*First test a small area where it won’t be noticed to make sure the vehicle’s paint job won’t be damaged or discolored.
Once you’ve removed the sticker, you’ll find a glue residue left on the vehicle. Keep heating that glue with a hair dryer and rub it off with mineral spirits or a vinegar soaked cloth. You can also try WD40 to get rid of the residue or use a commercial product like Goo Gone.
Once you’ve completed the job, wipe the area well with a hot, soapy cloth to remove the solvent (if any was used).