You may always change/cancel or add to your order anytime before 10 AM (on order day).
Step 1: Simply go to your group lunch link that your organizer sent
Step 2: Click on the "Bag" icon or "SEE WHAT EVERYONE IS ORDERING" on the top right of the page. (see attached-screenshot)
Step 3: Navigate to your order and make changes from there. (Do not forget to click "Confirm Order" after changing or canceling)
PS: You will not be charged if you cancel your dish before 10 am.