In the Day or Week view of the company calendar, use your mouse pointer to drag the top or bottom edge of the appointment box to extend or reduce the appointment time.  Note, this will not adjust the fee of the service.  If a different fee is to be charged, you may apply an adjustment to the invoice in the appointment edit screen or you can change the service by selecting the Service dropdown in the appointment edit screen.  There must be enough time available for the selected service to be booked. 

You may also set a service to be Variable Duration by going to Settings / Services / Duration & Assign and setting Variable Duration to "Yes" in the service edit page.  This will allow you to adjust the duration using the duration picker when booking or editing an appointment.