You can reverse a checked-in appointment in case this action has been made in error. All you need to do is to click on the reverse icon at the top right of your appointment window.
Please know that since the system automatically generates the invoice for the service when the appointment is checked in, reversing the action does not reverse the invoice already issued. That invoice will continue to be active and linked to the appointment unless you annul it by going into the invoice admin module.
Step-by-Step Guide: