You can add a note for an existing appointment any time before the Appointment has been completed.
Website
In the Calendar Page click on any appointment that is purple in color. On clicking a new dialog box will open where you can add a note and press Add Note button to save the changes.
Android
In the app click on the Appointment card to go to the details page. Scroll down to the Customer Note section. Click on the pencil icon to edit the note.
iPhone
In the app click on the Appointment card to go to the details page. Scroll down to the Customer Note section. Click on the edit note button.