change tempo in the drum track

Hi, I am a new user of Garage Band 10.1.4, just trying to get to grips with the basics. If I want to build up a song from the drum track, or metronome for that matter, how do I change the tempo within the track or song? eg. if a piece of music starts off slowly and gets fast at the end? Clearly the drums or metronome need to be changing speeds, but I can't see how to do that.