Thanks @crazycells . I I'm pretty sure it will become important to do that sooner or later.
probably sooner

7 years later... SUch ARROW-UP now exists, BUT it only edits the most recent message. Matrix/Element and some other apps now let the use cursor thru up line by line across individual messages., which I find myself continuously trying to do because it's an obvious action. It would be great to see this implemented.

Sidebar: Several years ago I bitched at @SlackHQ on twitter that I had been asking for a single character edit command (via email with support). You only got to it via the pull down selection. 3 years later, "e" was a reality. lol

I probably compose at least 400 post and chat messages a day on multiple platforms. Speed and simplicity of editing are very important to me.