when you've changed the directory so your list of songs appear in the main window, you can edit each field if necessary, once done you highlight what you want to tag (presumably all of them?), then you'll see 4 or so buttons with arrows on them on the top bar, the first one converts the tag to the filename, so what evers in your tag, it will rename the file to that. the second button changes the filename to the tag and so on
you can change how the tag and/or filename is displayed also. I tend to alter any tag settings first, then do the tag to filename option so it renames all my music files uniformly, then I run the filename to tag to update the tag information again (this step probably doesnt need doing but I always do lol)