You can do this from within the app or via your devices settings.
For more information on canceling a subscription see the apple support page
If you choose to buy an Rivr Pro or Originals Pro subscription, payment will be charged to your Apple ID account at the confirmation of purchase. The subscription automatically renews unless it is canceled at least 24 hours before the end of the current period. Your account will be charged for renewal within 24 hours prior to the end of the current period. You can manage and cancel your subscriptions by going to your account settings on the App Store after purchase.
All purchases are final. Rivr cannot issue refunds for purchases charged to your Apple ID. To find out more about refunds visit our support page.Request Refund from Apple