From you Order Confirmation Email scroll to the bottom of the email.

Click the “Edit or Cancel Order” link. This will take you to a screen to choose whether you want to cancel your order or edit it.

 Once you select ‘Cancel’ the order will be cancelled. If you select ‘Edit’ your items will be loaded into your cart for you to make changes.

You will then be able to add products or remove as you please and the system will automatically cancel the previous order.