You cannot edit an order once placed. But you can edit and reorder the content once it has been fulfilled.
To do this go to Orders and find the content item you need to change. Click the More options [...] menu to the right of the content item and select Edit and reorder. You will then be taken to the content details page for that item. Here you can edit any customizations. When ready, place your order as usual and it will be fulfilled as a new order.