Open Progus Store Locator and go to Plans.
The Plans page has a Cancel your subscription section.
The cancel button appears only when the app sees a paid current subscription.
Click Cancel current subscription to open the confirmation modal.
Confirming the modal sends the Free plan to the app subscription endpoint.
If no active subscription row is found, the app returns an error instead of claiming the cancellation worked.
If Shopify refuses the cancellation, the app returns an error asking you to try again or contact support.