Search for the lead where you need to edit an existing appointment, on the right hand side inside the lead you will find the appointment that is currently set for the Lead.
Once you find the lead with the appointment, click on the Title of the appointment, and it will redirect you over to the pop up where you can edit the appointment itself.
You are able to change and edit the Following information:
— User Attendees
— Contact Attendees
— Start at
— End at