You can edit the length of any appointment booked from the creation page or within the created appointment.

In the service section of the appointment creation window, click the pencil icon of the service(s).

Click the Duration drop-down to change the duration of the service.

Then click to save.

When viewing the appointment window, click the pencil icon next to the service

Similarly to the appointment creation window to edit the duration, click the duration drop-down to edit the end time of he appointment.