Microsoft has announced the availability of Windows 8.1 RTM version for both MSDN and TechNet Subscriptions. Any one with a valid MSDN or TechNet license can now login to the account in https://msdn.microsoft.com and start downloading the RTM version directly from the subscriptions download page right today. Microsoft initially announced the Windows 8.1 will be available for download in Mid October 2013 which has been lots of discussion with partners and developers who focused to develop apps targeting windows 8.1.
With this Microsoft finally decided to release the Window 8.1 RTM Version to the customer to upgrade their apps that target Windows 8.1 version. If you are a MSDN subscriber and having a Windows 8 OS running you can simply upgrade to Windows 8.1 with the update readily available or can download the OS image from MSDN and do a fresh installation from the scratch.
With Windows 8.1, for developers there are quite a few APIs and references opened for development which we can get through the Windows 8.1 SDK for development with Visual Studio 2013 RC that has been released today along with this release.