Admin --> Members --> Membergroups --> Edit Membergroups --> Post count based groups
Click on the Modify link on each of the post count based groups and change the name of the group.
Thank you so much for this, it was exactly what I was looking for.

Finally... I suggest that you read the online manual linked above, as all of your questions so far are answered there.
That's true, almost all questions are being answered in the online manual.
Problem is, such a manual, of course, is rather extensive and I lack the patience to go through the whole.
However being able to ask away specific questions in this forum (without any hurry) to get straight to the point answers a little later - honestly, it can't get any better than this for a noob like me. Besides this, there's this rule: to use as little effort as possible, in order to gain as much profit as possible. Once the forum is setup to our liking, I doubt we would have to look into the manual ever again.
There are a few other questions that I have though, where I have got the feeling the answer is going to be: "
That doesn't work". Instead of going through all pages of the manual unsuccessfully, I just ask here in the forum and you then tell me: "No way, GTFO".
That's fine by me
