For the first 7 days after you create a family everything is on — free and with no card: there is nothing to enter or cancel. 2 days before the trial ends you get a reminder; a tap opens “Subscription”.
Where to see it: the gear at the top right → “Settings” → “Subscription”. It says where you stand: “Trial — 5 days left”, “Subscription until 12.11”, “Free version” or “Full access”.