You can't edit a posted message. You have two choices:
1. With the original message displayed, scroll down to the bottom left of the message and click the link "Notify Administrator about this message?" In the message box type the word Remove. Then scroll down beneath the message box and check the small box beside "I am the original author of this post" and click the Send Alert button. It may take several days for the message and any replies to be removed. Then type a new message with the correct info.
2. An easier way. Simply post a follow-up message to the original message.