Your billing page will report that your account is managed by Apple when your subscription was purchased through the App Store on an iOS device.
Plans that are purchased through Apple can be managed through the subscription settings on your iOS device.
FAQ
Can Runway Support make changes to my plan if Apple manages my subscription?
No. These subscriptions are handled through Apple, which our payment processor does not have access to.
How can I instead manage my subscription through the web app?
To do this, first cancel your subscription through Apple. Once cancelled, log in to your account through the web app to start a new subscription.