If you purchase a Premium subscription, it'll renew automatically unless you cancel it. This way, you can continue to use these great features without having to worry about renewing them yourself!
If you do, however, change your mind, to avoid charges for the new subscription period, you must cancel it before the end of the current period. Cancelling your subscription depends on how you first signed up, but you can check out the steps relevant to you here.
Keep in mind that deleting your account (or the app) from your device, doesn't cancel your subscription.