How to edit the subject line of a message in Gmail? When clicking the return/down arrow I get a list of options as the following:
You can't edit an existing message, including subject line. All you can do is change the subject line when responding to a message. (This would spin off a new conversation thread.)
I do know that some third-party email clients will allow you to edit messages. You could always try using one of those, downloading your email, edit the messages you want, then re-upload them (or just use IMAP), but you'd probably end up with two separate messages and such things are beyond the ken of this site.
