If a quiz question needs to be updated or revised, that revision needs to be made BEFORE the quiz is taken. See Edit Questions used in a Graded Quiz for times when you can makes changes to a question after students have taken the quiz.
Revising or editing a quiz question
Go to the course homepage.
Find the quiz that needs updating and click on the activity link to that quiz.
Go to the Administration block, then click Edit Quiz.
Find the question within the quiz that needs to be edited.