Select the Tests & Quizzes tool from the Tool Menu of your site.

Select the Published-All option in the View drop-down to view the assessments that have been released to students in your site.

Select the Scores option from the drop-down menu for the assessment you would like to grade.

Alternatively, you may click on the number of student submissions in the Submitted column to view the submissions.

You'll be presented with the list of students.  Underneath the name of the student who needs to retake the test or quiz, click Allow retake?

Notes:

• An assessment must be Active (i.e. the due or late acceptance date must be in the future), for a student to be able to access it.  For instructions to edit assessment dates, see What are the Availability and Submissions options for an assessment?
• If the assessment allows multiple submissions, and the student has an incomplete submission with time available on it, the student can complete their available submission.  The instructor will not have the option to Allow retake for that student.
• Allowing a retake gives the student a new, blank copy of the assessment to take.

You'll be asked if you're sure you want to allow the selected student to retake the quiz.  Click Retake to confirm.

You'll be returned to the Submission Status page, where the student will no longer have the option Allow retake? underneath their name.  The student can now retake the quiz.