Body Switchers from Outer Space is a novella written by R. L. Stine and Nina Kiriki Hoffman. It was first published in 1996 and is the thirteenth book in the Ghosts of Fear Street book series.
Description[]
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!