Depends, is there a reason you're doing the past papers? i.e. studying for a test or studying beforehand in general? If it's for the first bit, then focus on the stuff you know. If it's the latter, just try as much as you can. It's good to expose yourself to a variety of different questions even from modules later on, so when you do get them later, you recognise and process things more quicker. Not saying to do them, but maybe trying to read + understand the question and check the answers to see how they got the answers even if you don't know the content might help in the long run