I have music without a genre tag or with incorrect genre tag.
Now is tehre easy way to let mp3tag find the correct genre based on the title , artist , album infomation, and let mp3tag replace or add the genre for the new one.
It would save so much time if that could be automated and get a more genre to the songs.
Not really.
This has been asked a couple of times.
BUT: if you have the tags set so that you get a good hit when you use one of the ordinary web sources then the found data should match the already embedded data and you can simply let the web source script overwrite the tags - you wouldn't see any difference - except that the genre has been added.
I need to figure out how to get that working with mp3tag. I need to look into that. It's at least possible i understand.
genres are a minefield ....
if you let an automated program do it you'll end up with literally hundreds of em 
@TagIt
And don't forget: If you ask 5 people about the genre for a specific song, you will get at least 5 different answers. Which one is the one matching your genre taste?
Genres are a highly subjective thing.
Genre-Example for Waterloo @ Discogs:
Genre: Rock, Pop
Stil: Europop, Pop Rock, Disco, Bubblegum
Which one would YOU choose?
I would just pick rock or pop.
In this case i use google for getting genre's so it would be POP.
Am very bad in tagging genre's right now myself.
At least not need to shame if i tag things wrong. 