Head to the "Upcoming" tab of your Orders page.
Click on the price of the order you wish to edit.
Scroll down and click the "Edit Dishes" button just below the price breakdown
Add or remove items from your cart to make the changes you desire
Once done with making edits, click the "View updated cart" button and follow the checkout process to finalize your changes.
Keep in mind, if you do not see the "Edit Dishes" option available, it may be past the cutoff time to make edits to the order.