Step 1: Have a look at the previous exams, in particular the exams from the last, say, five years. Are there questions or question types that are repeated every year, every two years, every three years? If yes, then it's likely that some of these will reappear in the next exam, so practice these.
Step 2: Do (almost) all questions fall into this category? Congratulations, you're done. (Well, probably. I remember one instructor who had asked the same questions for five years or so, and when I took the exam, he came up with a new set of questions. Bad luck.)
Step 3: Otherwise: Is there a significant number of questions in each exam that have not appeared previously, at least not in recent years? Then it is likely that these are assignments where you have to apply knowledge from the lecture to new problems, or where you have to combine several results from the lecture in some new way. In this case, you should expect new questions in your own exam as well, which means that you'll have to practice solving new problems. Note that this is very different from learning the answers to old problems. So you can practice using the exams from previous years, but you should try to solve them under the same conditions that the students from previous years had, without peeking at the sample solutions.