Unfortunately it's not possible to make changes to a DVD order after it's been placed.
If less than eight hours have passed since you placed your order, you can cancel your order, make any changes, then place it again. To do so, visit the Store tab, tap “My Orders”, tap the order you'd like to change, then tap “Cancel Order”. After canceling, please make any changes you'd like to your DVD and place a new order.
Please note that as DVDs are made to order, they can only be canceled up to 8 hours after an order has been placed.