You will need the Write Notes permission to perform this action.
Navigate to the person's profile whose note you want to edit and click on the "Notes" tab. Next, locate the note you want to edit in the table and click on the "Edit" button at the end of the row. You can also click on the date.
A pop up will appear where you can edit the details of the note. See this article for more information on the fields.
Helpful Tip
Notes of type "Archived Notes" and "Unarchived Notes" cannot be edited.