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.
The student will get a notification about your comments on their quiz, and they will see a revision task assigned under then "Tasks" side bar to the left of their dashboard:
When they click into that quiz, they will see a button to start the revision:
When the revised quiz is submitted, you will see it again in your teacher account
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.