Some editions of Windows 8 don’t support the free update to Windows 8.1. These include: • Windows 8 Enterprise • Editions of Windows 8 Pro that are installed by enterprises using KMS activation • Editions of Windows 8 that are installed using an MSDN ISO, and activated using multiple activation keys If you’re running one of these editions, you can't install the free update to Windows 8.1 or Windows RT 8.1 from the Store. http://windows.microsoft.com/en-us/windows-8/why-can-t-find-update-store