Appointo's subscriptions are managed under Shopify app ecosystem. There are 2 simple ways to cancel the subscription:
Please click on Appointo > Plans > Select Free Plan. Your subscription will be canceled and all the current bookings will be retained
You can uninstall the app by going to Settings in Shopify then Apps & Sales Channel and then Uninstall. If you uninstall the app, subscription will be cancelled automatically. Please be aware that once uninstalled, all products and subscription related data will be deleted.