First you need to enable this feature from Campaign >> General Settings
Now you will notice a new meta box named Send Message To Subscribers under post/package edit page:
Now, if you updated your post and send a notification to user who subscribed for this post, check Notify Subscriber button.
Fill out the form. When you click submit button, this email will be added to the email queue. Note that, if you didn't provide From Email and From Name, default value from Campaign >> General Settings will be used.