Upgrading from Windows 8.1 Preview to Windows 8.1
If you installed Windows 8.1 Preview as an update from the Windows Store
in Windows 8, you will be able to install the free Windows 8.1 update from the Windows Store.
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.
When upgrading from Windows 8.1 Preview/Windows RT 8.1 Preview to Windows 8.1/Windows 8.1 RT
, you will "Keep personal files only", but will lose your settings and have to reinstall all of your apps (desktop and Store).