To delete or edit your notes on a player, go to:
Settings → Gameplay → Player Notes → Edit Player Notes
If you only want to delete part of the note or edit it, highlight the player ID in the left-hand window. You can use the editing window on the right to delete or edit any information.
To delete all notes for a certain player, highlight the player id in the left-hand window, then press 'Delete'.
Close the editor once you finish editing to save your changes.