There are two ways to edit an existing appointment:
You can drag and drop the appointment box on the calendar to adjust the date / time or staff member.
You can also select an appointment on the calendar and use the pencil icon in the upper right hand corner to edit.
Both options opens up the appointment window where you can edit more details of the appointment. If the appointment's start time, or services are changed, you will have the option to send an updated notification to your client before saving.