Go to Messages and click Manage Messages.
Choose the message you would like to edit.
In the Action column on the left, click the link to Edit.
Your message will open and you will now be able to edit it.

If you see your message, but the Edit link is not available, your message was already sent. You cannot edit a message that has sent. You can however, clone (copy) the message and then edit it for a new mailing.