The only person who can edit appointment details, including the date and time, is the appointment organizer, i.e., the person who sent out the meeting request. You can send them a message directly via the chat feature, or by using the chat with participants button when viewing the appointment details and ask them to reschedule the meeting to a different time. Alternatively, you can decline the existing appointment, and send the other participant(s) a new appointment request with a note explaining the reason behind the new request.