Once you have published a quiz, the sidebar on your quiz page shows the Moderate Quiz link. By clicking the link you will move to the Moderate Quiz page, where you can view the progress of student submissions and view the number of quiz attempts each has taken. You can also grant students extra attempts, grant extra time for timed quizzes, and manually unlock quiz attempts.
Moderating the quiz lets you allow extra attempts for individual students as well as multiple students at once. This option also allows you to grant extra attempts for students who have not yet taken the quiz. For timed quizzes, you can grant extra time for an attempt. If your quiz options allow students to view results only once after each attempt, you can also let students view quiz results one more time.
You can also use these options to remove attempts as well. For instance, if you need to remove a student attempt, you can decrease the attempts accordingly.
Notes from Canvas Guides:
In the accordions below are a demonstration video of extending time for a quiz attempt, and deeper information from the Canvas guides: