You can always Uncheck the customer if you have checked them in by mistake
From Class Calendar
Go to your Dashboard
Select service from Calendar you want to make changes for
Select the name of the service from the Calendar
Uncheck the customer you have marked check in
From Check In
Go to your dashboard
Select the Check In
Uncheck the person you have checked In. You can change the date range and apply filters for the date and service you want to make changes for.