If you are using the Diary or Floor plan view to check people in, you'll need to bring up the left-hand side check-in panel by clicking the people icon at the top of your grid view.
> You will see that there is a ‘Check-in’ section with three boxes (as below), the field to the left shows the number of covers the booking has been made for, the second is your 'No Show' button and the third box is to enter in the actual number of guests that arrived
> To un-check the booking - select 'Uncheck Guest' from the drop-down menu in the third 'Actual Guests' box
> The booking will go back to 'not arrived' and will reappear in the Bookings list.
If using the Interactive Run sheet view, once you've checked in the booking, the Guests field will display the Guests vs Actual Guests - see screenshot below.
Click on this to bring up a drop-down list to choose 'Uncheck Guest'