You can do this two ways. You can use the change order to order a two pack, making two 32 slot servers, or you can use the change order to lower the slots on one to 32 slots, then order a new 32 slot server.
Both options would be done from: https://www.nfoservers.com/order.php
To make the order affect your current server, you'd select "This is a change to an existing server" and then select the proper identifier. The confirmation page would explain all the changes being made before you actually make the changes from your Payments tab.