GarageBand: how to slow down a song

[rda52]

this is my first time using garageband, so i dont really know what it can and cant do, so please accept my apology if i sound like a noob.

ok the problem is that i have a .mp3 song that i need to slow down, the song is for a theater performance. ive tried changing the tempo but it doesnt affect the track of the mp3 file only the master track, which doesnt slow down the song. how do i slow down the song, or what program should i use? thanks in advance.