You can only delete or edit your own messages. You must be logged in. Get your message on the screen. At the top of your message you will see an "X" icon and an "Edit" icon.
You can only delete your message if there have been no replies. Just click on the "X" icon and follow the prompts.
If there have been replies you can only edit your message, click on the "Edit" icon and change your message appropriately.
Be sure to click the "Submit" button when finished.