In addition to opening an appointment and changing the date and time there, you can also quickly reschedule an appointment from the calendar window.
Click and drag the appointment to a new day or time.
The appointment is rescheduled.
While viewing the calendar in month view, you can only drag an appointment to a new day, which will keep it at the same time. Clicking and dragging an appointment in the week view will let you change the day and the time together.