There are 3 ways to extend the booking:

1. From Bed View - quick Extend reservation

Right-click on the selected reservation and choose the Extend reservationoption from the list. Click on the number of nights you with to add to the reservation. 

2. From Bed View - Add selected nights

a) Select the nights like you do when creating a new reservation

b) Right-click on the booking to which you want to add the nights and choose Add selected nights to reservation option

    > This option allows you to add nights in a different room.

3. From Reservation Form - Change reservation dates

Open the reservation window, change the check-out date and save the changes:

See also: 

How to add new guest to the reservation