Fixing folder structure

After attempting to move my folder structure to Artist > Albums> tracks it noticed an error. It now goes Artist> Artist>Albums> Tracks

Is there a easy way to remove the first (or second) artist folder?

You could use Convert>Tag-Tag for _DIRECTORY with an absolute path, e.g.
Format string: d:\music\%artist%\%album%

That did it! Is there a good guide for the various different convert options and what they do?

The FAQs, HowTos:

and of course the help:
https://help.mp3tag.de/main_converter.html