I can see the genre, and look at it's properties, but I see no way to delete it.
edit: if it matters, I do not have any roms for those genres any longer. well, maybe a few, but I'll change those roms to have a proper genre first.
edit2: Just noticed you said rename, not remove. Not sure what that would accomplish. After fixing the issue that cause this, I correctly re-added the roms which created a list of valid genres.