Why not updating my preceding message?
Because so far I have found it impossible to find anywhere how to edit/correct/amend an existing by me created message. At least I could not find anything in the logical places I looked for.

Because that is deliberatly blocked to avoid spam which at one time was a problem. Some users are allowed to edit their posts because they have proven to be active and helpful participants and a such have proven to not be a spammer.
You can request the option to be opened for you but you will have to take the responsibility to use it wise. Otherwise you can wait for the Advanced Member status which I think automatically adds the edit option.