If you’ve got a Windows 8 device, you can download the update for free. To upgrade an older device, a download or an old-school DVD installation disc will set you back $120, $200 for Windows 8.1 Pro. The new software will be a full version, rather than just an upgrade — good for folks building systems from scratch, but Windows 7 users will have to reinstall all desktop apps, and Microsoft says the new version isn’t designed for XP or Vista users (though such folks can backup all files and do a clean 8.1 install)
