The exam is not scored until a) you submit the exam for scoring, or b) you run out of time. Think of it this way: you are given an exam booklet and a pencil. You sit down at a table and you mark what you believe to be the correct answer for each question. As you work through the exam booklet, you can go back to previous pages/questions and easily change your answers.
But! Once you hand the exam booklet back to the proctor, you can't get it back.
You will get your score from the exam proctor w/ a breakdown of test domains and the percentages you got correct in each domain. If you run out of time, the proctor takes the exam away and scores all unanswered questions as incorrect.