DON'T do this unless you absolutely cannot prepare the entire order.
If necessary, follow these steps:
1. Tap the "HELP" button in the top right of the order info screen that you would like to cancel.
2. Tap "CANCEL ORDER" and provide the reason for why you're canceling.
The customer will then get a notification that his/her order could not be fulfilled. You will not be paid for any orders that you cancel.