When using the Switchboard you can enter edit mode to update any information and related notes on a contact.
To add a note, you need to use the search bar to find the contact you would like to edit. Then click the Edit icon on the left of the contact's name. This will show the text field at the bottom for you to enter text. If the contact already has a note, it will be displayed and can be edited, or a new note can be added or created in its place.
Removing a note will unlock the contact.
After adding the new note you can lock the contact. Click the Edit icon again to leave edit mode.