For me it's sleep, but since u don't generally have much time during these stressful times, I suggest getting around 20-60 min sleep; that much time works for me, it may work differently for you tho.
The point of this is to let your mind rest by not thinking of anything; sleep works best in that regard imo. If you think that you might oversleep, either set a few alarms or just simply close your eyes and think about something less stressful.
Good luck with your exams, and I hope you get your stress levels sorted out ^^
Take a good and relaxing bath, take a nap (or sleep!). I usually do these whenever it's hell week, and read notes first thing in the morning when I wake up (usually around 3-6am) they said that reading when you woke up helps you absorb the information.
Eating helps in relieving stress too, but don't over-indulge because it may result to stress eating and weight gain.
But yeah, sleep is most definitely an effective technique (at least for me!)
Sorry, it's not manga related but how do ya'll deal with exam stress? Or stress in general?