Select a Subscription
From the Manage Billing screen select the subscription you want to update. Next select Edit Subscription toward the bottom.
Select Add Addons to view the available addons for your plan level.
Choose any addons you'd like and note the cost of the ones you've selected and choose the Add button to proceed.
You will be taken back to the Edit Subscription Details screen to view and confirm your changes. Once you've made all desired changes select Update Subscription.