This thing, on the top right in the sm editor.
If you were always wondering how to find the beat you're supposed to change the BPM at, well this is it. The trick is, to multiply it by 4. If you're doing it on a 32nd note or something, or you're going to have lots of tricky bpm changes at weird times, it comes in handy to have a calculator.
Stops are far more tricky, because they go by seconds. You should use cool edit for that.