Since Windows 8.1 is called an update for Windows 8, its obvious that Windows 8 product key should work for Windows 8.1 too but that’s not true! How to Upgrade to Windows 8.1 from Windows 8, Windows 7, XP and Vista? Other Windows version users need to buy a license of Windows 8.1. Almost all of us know that Windows 8.1 is an update for Windows 8 operating system and Windows 8 users can upgrade to Windows 8.1 absolutely free through Windows Store.