Exams can create a lot of stress and anxiety, which might interfere with performance. It is required that you take deep breaths, focus on your breathing for 5 mins and then resume to your studies when your mind relaxes. Try not to check the time repeatedly or count the number of chapters/pages/questions remaining as that might increase anxiety. Eat healthy fruits and keep yourself hydrated, avoid fatty/oily/heavy food items as they may induce sleep. Also make sure that you get enough sleep before you go for your exam. Wish you all the very best !