I tried to edit tags in my music library using gmusicbrowser (changing title, album, etc.) but it only changed the way the file is seen when opened in gmusicbrowser. The MP3 file is unchanged, untagged. I tried the "Update Tags" tool but nothing happened. Please, help! Thank you

Make sure the "Do not write the tags" option in the tags tab is not checked.
Otherwise, look in the advanced tag edition dialog that you get from a button at the bottom of the "edit song properties" window. The adv. dialog does not use the internal database, so what you see is what is in the file, check what values are in the tag(s).