You could select the files you want and choose properties (or add them all to playing now, playist, &/or search to select all) then the pull down menus will let you choose the info. It doesn't work on genre though, you have to pick that one. Wherever you see , that's where you can choose for all files.
Another way would be to highlight the files and then right click in a field (artist,album,whatever) and choose rename and then type it in and all will be set. If you do it this way and want to cancel out, hit esc..don't hit enter or click on the mouse somewhere else.
Last thing I know would be to drag and drop into a view scheme component. Like Genre/Arist/Album. Move the files to the Genre you want and it should update all files. Make other view schemes with different components and drag n' drop all day long.
This does one field at a time but for changing like rating on multiple files, this works. Doing it all at the same time would be nice but like Jim said, not at this time.