You can review questions and go back during the exam.
As you answer questions you have the chance to mark the questions for review.
when you reach the end you are presented with all the question numbers and the opportunity to go to specific questions or page through all the ones you have marked for review.
During the review you can change the answers or leave them and go forward or back through the list.
When you complete your review you can submit. Obviously if you run out of time the answers are submitted as they are at that time.