How may I edit the list of genres presented e.g. in the tag panel drop-down combo box? Thanks.
Under the Options.
Duh! 
Excellent - thanks Sebastian.
404 Error on the link. Couldn't find it otherwise.
Really would like to merge my Audacity genre list, which I can edit with my MP3tag list that I can't directly edit.
Ahem: that is the link to the help. Press F1 to open the help.
Or follow this link:
Yes, I saw that, but since the tags are defaulted to ID3v2.3, I thought that was fixed, so in most of my files only use that option. Do I have to rid of those files still using ID3v1?
I was also looking for the text-based file that generates it. Thanks.
As ID3V1 does not use the text but a defined number (id) set by the standard, all non-standard genres are mapped to "other". If you can live with that then you do not have to remove the ID3V1 tags.
You find a file "genres.ini" in the data folder.