Open an existing Notes file.
Click in the Content box for one of the notes and make some change.
You can't make the edit box lose the focus (and thus make your edit stick) by clicking outside the box somewhere in the whitespace area of the Notes file. You can click in another tile to effect this change, or on another clickable element in the Notes file window (e.g., another Note or a Post-It icon). But it's intuitive to want to click somewhere, anywhere, outside the note to make your change take.