Once a note has been signed, it is stored in the timeline or sent for approval (if an approval process is set up for your organization). If signed session notes need to be edited, you can unsign the note and create an addendum (a new note). You will need to resign the edited note.
To unsign and edit the note, find the session note in the timeline (using filters in the timeline will help locate the note). Then select, Unsign in the top right corner and remove. Removing your signature will restart the approval process.
Once you unsign the note, you can edit any part of it and then resign it. Now that you have resigned the note, it will be put back into the approval process (if there is one set up). To access all versions of the note, go to History in the top right corner and you will be able to see the original note and the revised version.