You can duplicate a quiz so that you can keep the original copy and then make changes to the newly copied version.
To copy a quiz, click on Quizzes:
Find the quiz you want to copy and to the right, click on the Copy icon. You can then name the new version of the quiz and click Copy.
The newly duplicated quiz will now show in your quiz list. You can add questions to this quiz, or edit it, just like any other quiz.