Plans and AI usage top-ups bought in the iPhone or iPad app are billed by Apple through your Apple Account; in the Android app, by Google Play. To change or cancel a plan bought in the app, open
Settings → your name → Subscriptions on iPhone or iPad (or the Subscriptions page in the Play Store on Android). Cancelling keeps the plan until the end of the current month.
Bought a plan on another device? Open the plan chooser and tap
Restore purchases.
Refunds for purchases made in the app are handled by Apple at
reportaproblem.apple.com (or by Google Play for Android purchases). Plans bought on the website are managed from the plan page on the website.