To delete a person's past attendance, just go to the customer's profile & click on the mistaken class attendance there. You can click through & delete the attendance from there...
An alternate way to remove a customer's attendance is by going back to the original attendance screen for a class.
Go back into the attendance screen for the class.
Click the 3 dots to the left of the customer's name in the attendance list.
Select the Trash Can delete icon and confirm to delete