If you make a mistake, or your schedule changes, you can change your answer to any confirmation request.
To change your response, navigate to your Upcoming Shifts page. Here, you can see all your Shifts and the confirmation status of each Shift.
Click on the Shift you wish to change your response for and then click "Do you wish to change your confirmation response?"
From there, you can Accept or Decline the Shift accordingly. If you are switching from confirmed to declined, you will have to enter a reason for declining the Shift.