As far as I know, the UML diagrams in the exam will based on the diagrams that appear in the textbook. I don't think it's going to be a massively complex diagram that will involve studying from other sources for it.
So take a look at the textbook and then if you're stuck on specific ones, put a question up here.
The reason that I say that is because the field of UML is rather large (see the
page at wikipedia). That's why I'd suggest starting from the textbook.
Also, you can get an idea of what types of UML questions they will ask from the past paper.