Volume license customers and subscribers to MSDN Plus and TechNet Plus were initially unable to obtain the RTM version upon its release; a spokesperson stated that the change in policy was to allow Microsoft to work with OEMs “to ensure a quality experience at general availability.” However, after criticism, Microsoft reversed its decision and released the RTM build on MSDN and TechNet on September 9, 2013.
Unlike previous Windows service packs, Windows 8.1 is obtained as a download through Windows Store for users of retail or OEM copies of Windows 8 and RT. Users of Windows 8 Enterprise edition, volume licensing customers and MSDN or TechNet subscribers must obtain Windows 8.1 as new installation media, and install it through the traditional Windows installation process. In any case, versions of Windows 8.1 distributed as ISO images do not accept Windows 8 product keys.
Please we don't provide serials or keys...and don't put any one in a comment below...Thank you.