Click on the appointment in the calendar. On the left side of the screen you see appointment details. Below the details you see the avaiable actions. Choos "edit".
In the next step you can change the appointment details. You can choose to inform the customer via email about the changes you made.
Sometimes it's better not to inform the customers via email, for example when you only wnat to change the room needed for the appointment. Choose "book now" to confirm the changes.