As an instructor, you can moderate a published quiz to do things like provide time extensions or add additional attempts. using the following steps. Moderation can be done after a quiz has been published and before or after students begin taking the quiz. To moderate a quiz:

• Go to Quizzes and click on the name of the published quiz.
• Click on the Moderate This Quiz text on the right-hand side of the page.
   

• Option: Search for a specific student using the Filter button.
• You will be able to see information regarding Attempt, Time, Attempts Left, and Score  for each student. 
   

• Click on the pencil icon to add extra attempts or extra time on every attempt. If your quiz is now locked (passed the availability window) click on the tickbox to Manually unlock the quiz for the next attempt.
• Once you have made your changes you would then click on the Save  button.
   

You can use Moderate This Quiz to monitor students taking the quiz real-time.  More information is provided in the section Add Extra Time on Current attempt on the following webpage - https://community.instructure.com/en/kb/articles/661034-once-i-publish-a-timed-quiz-how-can-i-give-my-students-extra-time.  
Note: The quiz availability dates supersede extra time given, meaning that if the Until date and time passes and a student is taking a quiz with extended time, the quiz will auto-submit, even if the time extension has not yet expired.