From Settings > Account, in the "Subscription" block, a button with a crossed-out calendar icon lets you cancel your subscription. It only appears if your subscription is paid and no cancellation is already in progress.
Clicking it opens a confirmation dialog showing your access end date and a summary of the consequences (sites or team members lost, if applicable), before you confirm anything.
Once cancellation is confirmed, this button disappears and is replaced by a line of text showing your access end date.