If you saved your designs when you placed your order, you will find your saved designs in your personal space, saved creations tab > ordered creations tab.
You will then be able to view your saved designs, modify them and re-order them.
If this is not the case, you will find the "Re-order" button in your order details, under the "Orders" tab. A request will then be made to generate a new order using the same print file as your initial order.