Body Switchers from Outer Space (Ghosts of Fear Street #14) by R.L. Stine, Nina Kiriki Hoffman

Come back with my body! Will "the Spill" is a total klutz. He'd give anything to be Chad -- the most popular kid in school -- just for one day. Chad never trips, or falls, or drops anything.

Then Will gets his wish. Chad says he's got a body-switching machine. And he wants to switch bodies with Will!

Will decides to do it. And it works. He loves being Chad -- until he finds out Chad's horrible secret. And realizes he's trapped in Chad's body forever!