These instructions are for users that are subscribed via fieldmargin.com. If you subscribed via in-app purchase you can find instructions for Apple and Android.
Go to account.fieldmargin.com
Choose the farm you would like to downgrade
Click 'Manage'
Choose 'CANCEL RENEWAL'
Confirm your cancellation
You can cancel your farm’s subscription at any time. You will continue to have access to premium features until your renewal date. At this point your farm will revert to the free plan and you will not receive any further charges.