For those who want/need to change the email subject of a received email message either because the sender left the subject line empty or just to better organize the messages, the process to edit the subject field is simple but not readily evident for everyone.
In order to change the subject of a received email message, the message must first be opened. With the open message, the message header must be in “expanded view” which is control by the very small arrow to the extreme right of the email subject field.
If the email header is not expanded the arrow will be in the down position where you will not see the TO email address
In order to edit the email subject, click in the arrow which will expand the message header at which point the arrow will appear in the Up position
The setting for the message header persists until it is changed again. In other words, it only needs to be done once until you no longer want expanded message headers.