How do I let a student take a quiz again

If a student takes a quiz and wants to take it again for whatever reason, you can delete their original attempt to allow them a second chance.

Go to Quizzes.
Select the Grade Quiz icon (ruler with a green checkmark)
Select Display Options.
In the popup window, select Allow Reset.
Select Save.
Find the student. Select the checkbox in front of her name.
Select the trash can at the top of the list of names.
Select Yes in the popup confirmation window.
That attempt will be deleted. As long as it is still within the timeframe for taking the quiz, the student will be able to take it again.