Hold Select, press touch strip.
The Select button is at the bottom right of the Push 2. Just hold it down, and press the touch strip until the display says “Touchstrip Mode: Modwheel”. To switch it back, do the same until it says “Touchstrip Mode: Pitchbend”. Simple.
I wasted several minutes googling for a textual description of how to make the Push 2’s touch strip act like a mod wheel instead of pitch bend, failing to find one, then having to watch a 5-minute video in order to find out something that could have been described in 5 words. So I created this page in the hope it will save someone else the bother.