Rated 4 out of 5 stars

I finally figured out how to use this this add-on to do what it says it will do. Here is how.
First, if you want the edited subject to appear in the message you need to change the option:
Click Tools in the menu bar, in the pop-up menu find Add-on Options, hover over it with your mouse pointer, in the sub-menu click Edit email subject, in the dialogue box that appears uncheck the check box.
How to change the subject:
In a folder view: find the message that you want to modify the subject line of, with the mouse pointer hovering over the message information right click on it, in the popup menu find the Edit Email Subject menu item and click it, in the EditEmailSubject dialogue box edit the subject of the message, click OK.
While reading the message: Right click with the mouse while the pointer is anywhere in the message body, in the popup menu find the Edit Email Subject menu item and click it, in the EditEmailSubject dialogue box edit the subject of the message, click OK.
This add-on does what it claims to do, I give it 5 stars for that. For its lack of any documentation explaining how to use it I give it one star. Since I had to figure out how to use it myself, overall I give it 4 stars.

This review is for a previous version of the add-on (2.1.0).