Cannot modify reservation for guest leaving early

[Francisco1136]

A guest left one day early and he suggested me to modify the reservation so we can have one additional hosting day. 

Great! I go to the modification option and it displays a calendar from which I have to select both arrival and departure dates, but since the arrival date was some days ago, the calendar is greyed out and I can't select the same arrival date, therefore, I just cannot modify the reservation (unless I select today as arrival date, which is useless, since today it is also the departure date)

In this context, how could I shorten 1 day the stay of a guest?