Once in your Subscription Portal, click the "ADD PRODUCT" button.
You will see the available subscription products that you can add to your current subscription.
After clicking on the desired product, you will have three options:
QUANTITY - Choose how many you would like to add to your subscription.
NEXT SHIPMENT - This is the date that your new product will start being added to your subscription. Leave unchanged to begin on the next billing date.
ADD TO SUBSCRIPTION - If you have more than one active subscription, you may choose which one you'd like to add your new product to. In most cases, you'll leave this unchanged.
Once you add a product, you can easily cancel it at any time by choosing the "Cancel" link next to it in your Subscription list. See cancel guide here.