"If you installed Windows 8.1 Preview by booting from ISO media, you can update to the final version of Windows 8.1 Pro using the Windows Store, but your system won't activate until you buy a Windows 8.1 license and product key. You can do this by following the instructions when you activate your system, or by purchasing a DVD of Windows 8.1 Pro. If you don't want to buy a copy of the Pro edition of Windows 8.1, you should return your PC to its previous operating system before installing Windows 8.1."