You can change your payment method details by going to Walkens's website and choosing "Subscription" from the settings menu. If you signed up for a subscription using a third-party account (e.g., iTunes, Google Play, or similar account) as a payment option and plan to cancel your subscription at any point, even during your free trial, you must do so through said third party, including by accessing your account with the relevant third party and shutting off auto-renew or unsubscribing from the service.