Notes can be added to Persons, Couples or Parent-Child relationships.
View notes for a person/couple/parent-child using

Create a note using
Click on to complete the transaction and create note.

Once created the notes appears in the list, and can be edited.

Creating Notes
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After editing, click to upload changes to FamilySearch

Notes can be viewed on FamilySearch using

View changes on FamilySearch
(Click on the image to enlarge)