If you you open each audio event in the editor, there is box with a tempo in it at the top of the editor. You need to change that to what the tempo is currenty if it isn't already, then there is a button at the far right of the editor with a musical note on it. If you press that it will light up blue and put the event in mucial mode, which will effectively tie it to the tempo of the track. You will need to do this to every audio event in the project. But not to midi.
You also need to set each channel to musical if it isn't already. The actual channel itself should have a button to do this somewhere. The button will either have a musical note or a clock on it depending if its in musical or linear.
Once you've done all that, you should be able to just change the tempo and everything will stick to it. This works quite nicely if you have lots of tempo changes in a tune. However it might also come out sounding a bit unatural if you overdo it.