Within your teacher account, you can allow a student to retake or revise a quiz.
From your Dashboard, click on the row of the student whose quiz you want to remove or allow revision:
Locate the quiz, then click the Options dropdown next to the score:
Click Remove Quiz to delete the quiz entirely, which will allow the student to retake the quiz with new questions. Click Allow Revision to allow the student to make edits to their answers and resubmit it for a new grade.