In the transport window there is a box that shows the BPM. Change it here. If it reverts to the original setting when you hit play, click on the Word "Track" next to the BPM display, which will change to "Fixed" (Good if the whole song is to remain the same tempo throughout). Or, leave it set to Track, and open up your Tempo Track and manually adjust your tempo graphically (You need to use this method if you are going to change the tempo within the song). Also in the top space of the Tempo Track you can insert meter changes too (i.e. 4/4, 6/4, 2/2 etc. (Or you can do it in the Transport window, if your using "Fixed")).