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 a 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 it's 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.