It’s also good to get into a routine before an exam - I always tried to relax in the 5 mins before an exam, talk to my friends and get my stuff ready. Prevents me from cramming and panicking. During an exam if I felt myself getting stressed I’d stop for a minute, take a few deep breaths and take a sip of water.
But most important is knowing the material and doing practise exams - it really boosts your confidence.
Know your stuff, revise enough and know you know it! Remember to breathe in the exam hall! I am a christian and I would find praying is helpful and I understand praying is not something everyone wants to do. Eat enough even if you feel ill, bring cool water in to shock you if you begin panicking and ground yourself. I once massively panicked in an exam and I wasted a third of ym time. I shut my book and just breathed, I knew this exam is not the end of the world, I looked at the staff, fellow students, generally distracted myself and wouldn't allow myself to open the paper for 5 minutes. Once I had chilled out some, I started again, finished the exam just in time and it was fine!