If your instructor has enabled the option for your assignments, and it is before the assignment's due date, you may notice a "Reopen" option for your assignment attempt.
Reopening a Completed attempt un-submits the attempt and places you back at the beginning of the assignment while retaining all of the data and progress from the previous session. This feature is a great tool to help improve your score as once you are done reviewing your results, you may reopen the attempt to make any desired revisions.
Please visit this article for instructions on how to Reopen your attempts.