Within your teacher account, you can allow a student to retake or revise a quiz, and resubmit for a new grade.
On your teacher account, go to your Data page and select Quizzes. Choose Feedback, which is located beneath the quiz score:
To request revisions, you must first leave a comment for the student on one of their quiz answers.
Once you submit a comment, a checkbox will appear allowing you to request a revision.
To delete the quiz entirely, select Edit located under the quiz score, and click Delete Quiz. This will allow the student to retake the quiz with new questions.