To cancel your Structured Pro subscription, go into the ⚙️ Structured Settings. Here, tap on the first entry that reads Thanks for using Structured Pro. Now use the Manage Subscription button and cancel your subscription in the popup.

Alternatively, you can manage your subscription in your device settings. Open your Apple ID page by tapping on your name in the settings app. Then open the entry Subscription and select Structured. Here, you can cancel your Structured Pro subscription.

To learn how to request a refund, click here