A bit late for people doing the morning session but for those doing the afternoon one, my tip is to stay calm. UMAT is really a test of mental strength and is really straining mentally. If you get stressed you won't be able to think as clearly. So no matter what just stay calm and give it your best shot. It's not the end of the world if you don't do well as there is always the following year.
I am saying this from experience which helped me get a UMAT score of 85+ (although i didn't follow up with doing Medicine). Basically the night before UMAT I was essentially playing games, going out to movies with friends just to forget UMAT is on. It really helped me focused during the exam.
All the best to everyone!