Sign in at chatgpt.com, open your account menu and select Settings.
Select Billing.
Under Cancel plan, select Cancel.
What happens
Charges stop
Paid features stay on until the end of the current billing period.
Refund
Cancelling doesn't refund past charges. That's a separate request.
The catch
Cancel at least 24 hours before your billing date.
Subscribed in the iPhone app? Cancel in iPhone Settings → your name → Subscriptions → ChatGPT. Deleting your ChatGPT account does NOT cancel an Apple subscription.
Subscribed through Google Play? Cancel in the Play Store under Subscriptions.
Uninstalling the app doesn't cancel.
Billing controls missing? Try the same account in a different browser.