Like with any app, you must manage your app subscription and payment through your provider — Google or Apple.
To subscribe to the paid Premium version of the app: Click on any Premium content in the app, and you will see a subscription screen appear. Note, the locked Premium content comes with your paid Premium subscription. Just click on the content you want to view and press play!
To unsubscribe from the app: Visit your Google or Apple account, in the same location you manage all of your app subscriptions. Click on My Subscription, located under My Account; this will open your app subscriptions screen.
If you have any questions about your payment, please reach out to your app provider and/or bank.