This procedure will revert the whole Notes database to a previous version. It's not possible to restore individual notes.
Triple-click anywhere in the line below on this page to select it:
~/Library/Containers/com.apple.Notes
Right-click or control-click the highlighted line and select
Services ▹ Reveal in Finder (or just Reveal)
from the contextual menu.* A folder should open with an item named "com.apple.Notes" selected. Quit the application if it's running. Move the selected item to the Trash, then restore it from a backup that predates the unwanted change. If you back up with Time Machine, enter it and select the snapshot from which you want to restore.
Log out or restart and empty the Trash.
If you synchronize Notes with iCloud or another network service, the notes you restored may be immediately deleted after you restore them. In that case, temporarily disconnect from the Internet, for example by turning off your broadband adapter, and restore again. Copy the contents of each note to a document in another application, such as TextEdit. Reconnect to the Internet. If the notes are deleted, recreate them from the TextEdit document. They should then synchronize to the network.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select
Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You won't see what you pasted because a line break is included. Press return.