To remove or "uncheck in" a person who was accidentally checked in to an event:
If you have check out enabled in the event settings, check the person out and then click the box again. You'll get a pop up asking if you want to remove the check in record and you can click to confirm.
Select the "Events" tab from the top left. Click the event for which you'd like to remove check-in data. Click "Check In."
If you do not have check out enabled in the event settings, simply check the box again and it will "uncheck in" the person.