The scores for quizzes are stored in hidden data in each user profile.

You can clear out quiz scores using a feature available only to administrators. Go to Explore mode, then click "Admin Functions" tab below the record. Then click on the "Clear Quiz Results" option. As an administrator you will see some additional options here compared to what you would see if you click "Try Again" on a quiz:

This pertains only to the quizzes on this specific article. You can select "For All Users", or select a specific user to clear the quiz results for. You can also specify a date, and it will then only clear out quiz results if the quiz was taken before that date. This can be useful if you update the article significantly and want users to take the quiz again. (Note, though, changing a quiz will invalidate any quiz results related to that quiz, so they will be forced to take it again anyway).