Open Billing and choose Manage subscription to change or cancel your plan. Buying in the iPhone and iPad app opens soon; a plan bought there will be managed in your App Store subscriptions.
Plan changes take effect straight away; a downgrade sets your new allowance from the next renewal. After cancelling, your books stay readable, and pack pictures stay usable.
We don't issue prorated refunds for part of a month unless your local consumer law requires it.