Hi
I don't understand your question. You are looking at a student's submission, right? Then you are not supposed to change his answers. You can give feedback, informing the correct solution, if you want to do that. But you can't simply change the answer!
BTW, the whole quiz is blocked, once a student answers it. The teacher can delete all submissions though, if he wants to redo the quiz.
BTW, there is a dedicated forum called Quiz.