You can mark a question "Marked For Review" if you are not sure about the answer you chose. By clicking on this option, you can review the question later. From the right side of the screen, you can actually check the status of the questions as in whether you have attempted or not. You can also check the questions that you have marked for review. But once you submit the test, you cannot go back and re do the test. You just get the results. You cannot revisit the test once you have completed the test and clicked on "Submit the Test".