Applying bottle credits/rewards points towards subscriptions
First login, then click "Manage Subscriptions" on the first page after logging in. Next click the green "Apple Discount" button below your subscription. Copy a code from the reward app in the bottom left hand corner of the screen.
Adding Products to your subscription
First login, then click "Manage Subscriptions" on the first page after logging in. Next click the green "Edit Products" button below your subscription. Search for the product you want and then click add product!
Skipping and Cancelling
First login, then click "Manage Subscriptions" on the first page after logging in. Next click the green "Skip" or "Cancel" buttons below your subscription.