To edit text of a note that is already created, simply double-click the note and change the text just like in Notepad.
To edit note color, font or to change its other settings, click on the spanner icon in the upper right note corner.
Here you can change the note color, font, transparency, make it fit to text and if you select the "Always on Top" checkbox, the note will always appear on your desktop overtop all your open windows.