Get enough sleep, eat good food, and keep moving.
It's really important you look after your physical health when studying for exams. Make sure you're stopping to eat regular meals and try to set aside 30 minutes a day to do some exercise, even if it's just going for a walk. Having enough sleep is also really important.
If you're really stressed
If you're finding things tough, ask for help. It's your teacher, lecturer or tutors job to help you understand the subject, so if you're not understanding stuff, tell them and they should be able to help. If your study load or exams, assignments or assessments are driving you mad, then book in to see someone at Student Support Services who can help with that as well.