You can edit the files directly in the folder - let MP3tag open and load the files of the folder.
If you have already got editted copies of your files, move them to the music folder.
You have to let WMP do its update process before you see the result. The quickest way is:
Delete the files from the WMP library (not the file system).
Trigger Extras>Enhanced>Restore deleted items
This updates the tags in the library if you have not changed the filename.
Sorry - but as I've only got a few hours experience I need to ask you a couple of questions if that's OK:
1 - Which folder are you referring to, please?
2 - <If you have already got edited copies of your files, move them to the music folder>
Thinking about it, ideally I'd like to back up my existing Music folder and then empty it. Then copy all the (corrected) files from the Mp3tag Directory into the Music folder and then do the WMP update.