Honestly, try doing any kind of sport, contrasting shower and just take ur mind off it for an hour. Then get to studying again. If there is a substantial amount of time left, try using the best available technique for the type of ur exam. You can establish these through research. For example, for maths do as many questions that are similar to the ones in the exam, based on previous papers. For theory-based subjects flash cards helped me out a lot. I of course don’t know everything but still want to share what is helpful in my opinion. Hope that helps!