this gets brought up ALL THE TIME!
ok here we go again,......
in short, YOU CAN'T. (notice the period?)
99% of the time the decals get placed directly ON the basecoat, and then get clearcoated, so there's NO CLEARCOAT under the decals, which means when you remove the decal you will rip off alot of that basecoat (if not all), which leaves a defect, which when clearcoated over, will be VERY NOTICIBLE.
you can certainly try, but I (after nearly 30 years at this) I only had ONE work where I could remove JUST the topcoat, then the decal, and re-clearcoat it, and have it be a transparent (un-noticible) repair, but that was a factory powdercoat, that had been just shot with spraycan paint with NO cleaning or sanding or anything, I basically used a scotchbrite pad to remove the flaky peeling clear, and then the home made decals, once the powdercoat had all of the spray clear off it, I scuffed it some more, and clearcoated it.
(if yours is like that, it MIGHT work, IF your careful)
but more than likely if you REALLY want to do this, you'll have to strip the frame, and then re-paint it.
sorry for the hard news.