If you're only hours away from an exam, there's little you can do at this late stage anyways. You might be able to cover a chapter or two that you're somewhat weak on ...but either you know it or you don't at this late stage. Don't stress, and get a good night's sleep. If you stay up all night cramming for that 10-20% you don't already know, you jeopardize drawing a blank on the 80-90% you do know due to being fatigued. Good luck, BTW.