Right now
I would do some light review of things that you confuse or forget and maybe look over some equations. I would also go over your test strategies (for example, will you read the whole RC passage, or just skim the passage). I always find that writing out your plan for the test will help you feel prepared so that you don't panic tomorrow. Also, prepare some review questions for the morning (see below)
Later today
You should also plan out your meals for tomorrow. Decide what you are going to eat for breakfast and take something with you to the exam. Personally, when I am stressed I can't eat- so I always like to just drink a Boost or Slim Fast Shake. They are a good source of quick calories to that you don't pass out during the test. I would also take one of them to the test with you for the break. Otherwise- enjoy the day. Watch a movie or go for a run. Try to get a good night sleep (i know it's easier said than done).
In the morning
This is what I always do before a big test- Before I leave the house or once I get to the exam building, I look over questions that I have already done and FULLY understand- 15 questions should be plenty. Since bio is the first section, it seems logical to go over some bio questions. I personally find that doing this gives me a boost of confidence. I try to not bring any exam materials to big tests- but if you must, limit yourself to one or two sheets of things you know inside and out. I also always get nervous if people around me are talking about exam material so I ALWAYS bring my ipod so I don't have to listen to them. So much of this test is your confidence- so for the next 24 hours do everything possible to boost your confidence level (even if it means reading The Little Engine That Could

).
During the test
Pace yourself, if you don't know the answer- move on. If you know how to do a problem but it will take you some time to do- skip it. If, for some reason, you start to panic- take a deep breath and move on. Do not dwell on sections that you didn't feel good about or on answers that you didn't know- this will only hurt you.
Just take it easy today- don't attempt to cram. If you have other questions or want specifics that I would review PM me. Good luck tomorrow! Let me know how it goes.
🙂