You can edit the notes created by you and also delete them if you no longer need them.

Here's how you do it, 

1. Visit the details page of the lead or the contact whose note(s) you'd like to edit or delete.
2. Click Notes. You can find all the notes created by you and by others in your team for that lead or contact.
3. Select the note and 
1. Click the Edit button, if you'd like to edit the note and click Done to save the changes. 
2. Click the Delete button, if you'd like to delete the note and hit Yes in the dialog box that appears.