First, you can only edit a message if you entered it as a registered user.
To edit the message, click on the text reading ``[Edit]'' next to it. (If it's not there, then you didn't authenticate yourself when you created the message, and so you can't edit it.) Fill out the form (remembering especially the Password field), and select the ``Change post'' button.
You cannot delete a message; the best you can do is remove all the information, leaving just your own name.