If you purchased a premium subscription through the iPhone or iPad apps, you must manage it from your iTunes account. You can access your subscriptions from the settings in your iOS device and then select your Apple ID. Your subscription will auto-renew for the same plan, price and duration as your original purchase unless you turn it off in your iTunes Account Settings at least 24 hours before the end of the paid period. Learn more about subscription management here.
If you aren't seeing your Premium status reflected when using the app but you have subscribed through iTunes, please try logging out and then logging back into the app.