Simple! Just follow the steps outlined below and you'll be all set :-).
1. Click here to log in to your account.
2. Once logged in, click the "Manage Subscriptions" link in the panel on the left-hand side of your screen.
3. From here, you can edit your current subscriptions by clicking the "Edit" button on the right-hand side of each order and you can add products to your current subscription by clicking the "Add Product" button at the bottom of your screen.