newbie: mysql statement

I want to update the most recent record (based on the timestamp in field posted) where the parent field == a specified value (in a table called header). I tried the following mysql statement: "UPDATE header WHERE parent = '$this->postid' ORDER by posted SET parent='$this->parent' LIMIT1"; but apparently you can't use ORDER in an UPDATE statement. If I take order out, the statement works. That being true (and please correct me if its not) how can I ensure that the newest record is the one being acted upon? Thanks.