This is because the announcement has already been delivered. You can edit announcements that have not yet been delivered, but you cannot edit them once they have been sent.

You can set a future delivery date on the announcement, so that wil give you a period of time to review your message and make any changes before the system delivers it.  After a message has been sent, it would need to be deleted and recreated if edits were desired.

Thank you
-WebCampus Support