I change tags in my mp3/m4a files using Tag&Rename but it seems that iTunes don't support this changes - in iTunes I still see old information.
iTunes have its own database and look into file tag only when you add it to iTunes and when you play this file, so it will update tag changes while playing or you can do the following:
1. Select in iTunes all your files.
2. Right click on it's and select "Get Info" - this will open iTunes multi file tag editor.
3. Don't check any checkboxes or change anything - just press "Ok" button - iTunes will reread tags from selected files after this.