Overrides can be used in both the Quiz and Assignment activity to permit more attempts, change open/close date/time windows, and change the time limit.
To add a user override to your existing quiz:
Note: if you do not alter one or more of these settings, the settings will remain at the original quiz defaults. For example, if the quiz is closed to all students because the quiz’s original close date/time has passed and you do not adjust the override’s close date/time, even if you give a specific student a second attempt, they will not be able to access the quiz.