Your valid Premium subscription can be restored in the app at any time. If the Premium service is not activated in the app, please activate the Premium purchase screen in the app by tapping on a Premium feature within the app (marked with a lock). Then tap the "Restore Purchases" link (in the middle of the screen) and select the Google Account that was used for the update. If Premium is active, the screen will disappear and the Premium service will be reactivated.
Please note: The Google account that was activated when you purchased the Premium service must also be used for the recovery.