Microsoft has started releasing Windows 8.1 and Windows RT 8.1 to hardware partners ahead of the public October 18th release; however, the update has already leaked online.

Thanks, in part, to customer feedback and an unparalleled level of collaboration across product teams, Windows 8.1 is a significant update. We have delivered in a very short time an update to the OS that will bring an even greater unified experience for our customers. As we consider the code we just handed off, and the new intuitive and fluid computing experience it provides - anytime, anywhere, across all devices – we’re confident we made the right bet in continuing our vision and following through on our commitment to rapid innovation and responsive engineering.

Microsoft has indicated that the release is only for hardware partners and will not be available any earlier to commercial customers or MSDN subscribers.

While our partners are preparing these exciting new devices we will continue to work closely with them as we put the finishing touches on Windows 8.1 to ensure a quality experience at general availability on October 18th. This is the date when Windows 8.1 will be broadly available for commercial customers with or without volume licensing agreements, our broad partner ecosystem, subscribers to MSDN and TechNet, as well as consumers.

Despite not being available for nearly two months, WinBeta is reporting that the 64-bit release has already been leaked.

"The Windows 8.1 Enterprise RTM build is native Chinese, we assume you can translate this build with the build in language+region settings however we are yet to confirm this. We do not recommend you download leaked builds, however as this is the RTM it is finished code, do what you will with it, but don't say we didn't warn you."

You can learn more about the new features in Windows 8.1 here.