By 'sticker' do you mean bumper stickers, or graphics?
Either way, use a heat gun (or a good high-wattage hair dryer). Work a few inches at a time -- heat it and peel it, heat it and peel it. Any sticky residue remaining can be removed with WD-40. You'll undoubtedly have some 'ghosting' where the stickers were, which could require some professional buffing, or at worse just leave it alone. Depends how long they've been on.
. . . never confuse education with intelligence, nor motion with progress