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
Click the "View updated cart" button to view your order details.
Towards the bottom of the screen, you will see a drop down menu for available delivery dates, and then times of day for the delivery to take place.
Choose the date and time that works best for you and click "Save".
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.