The documentation below demonstrates the process involved in resetting a user's quiz attempt.
Log into your course and from the Assessments drop-down menu click on Quizzes.
From the drop-down menu next to the quiz click Grade. A list of the students and their attempts will appear.
Check the box next to the attempt(s) you wish to reset.
Click on the trash can icon, as shown to the left.
A new pop-up will appear to confirm the deletion(s). Click Yes to confirm.