In Quiz, add the modified question from the Question Bank and remove the old question.
The old question will stay in the Question Bank untouched and may exist in some other existing Quizzes, so you can leave it there if there’s nothing wrong with it.
If you do want to remove the old question from the Question Bank, you can. But it will only be removed from the list - actually, it will stay forever in the system under the checkbox ‘Also show old questions’.
What happens if you change the correct answer, but the Quiz has closed and the students have already been graded?
The changes will show in the Question Bank.
The past quiz result will remain untouched, even though the correct answer has been changed.
The ongoing quiz will be marked with the new correct answer.
You can also duplicate the question in the Question Bank, change the answer and add the new one in. This way, you leave everything in the past quiz as it was.