You can quickly change a patch's tempo inside MainStage. If you have created a tap tempo button inside Layout mode, you can map it to a button on your controller, and tap it in while playing live.
You can also designate the tempo in advance.
- Select your patch in the patch list.
- Go to Patch Settings (in the Inspector) > Attributes
- Change your Time Signature / Tempo
- Save (⌘S) so it "bakes in" your settings