A broadcast can only be changed before it has started to be sent out. While the broadcast is in the “waiting” state, its Actions column in the queue manager has an Edit link. When you click it, you can change all the parameters of the broadcast or the message text. While you are editing the broadcast there is a possibility that the auto.php cron task runs and the broadcast starts being sent in the background. If this happens then the changes that you are making will not be able to be saved. If you have made a mistake in your broadcast then you may want to delete it and re-create it instead of risking the broadcast being sent incorrectly.