Hello @Dean-and-Sue0,
You can first send the guest a message saying that you are going to send a reservation change to include hte extra date and that they must respond by accepting it.
Then you can go to your reservations by clicking on "Host" and then "Your reservations".
Then find the reservation you wish to change and click on "Change or cancel" for it.
On the next page click on "Change Reservation" and then select the new checkout date.
Then click on "Submit".
The guest must then select "Accept" for the change to be effected.
Steve.