Once you activate a free trial through the Apple App store, you will need to cancel at least 24 hours before the trial ends to avoid being charged for the annual subscription.
Please note, you will need to cancel through the Apple APP store settings on your device.
Open your iPhone settings
Tap your name at the top
Tap the subscription you want to manage, in this case fieldmargin
Tap Cancel Subscription
If you do not see fieldmargin listed, this means you do not have a free trial activated on this Apple account.
Tap the 'Cancel subscription' option and follow the on-screen instructions to confirm cancellation.
If you run into any trouble, this apple article, includes additional details for cancelling your free trial.