A preview version of Windows 8.1 has been available since June, and if all goes to plan, word on the web is that the final release will roll into town in October. That's when the general public will be able to nab the download via Windows Update. Prior to that, OEM system builders will receive the update as early as this month, and it's possible some Windows 8.1 machines will show up in retail in September.
The headstart for OEMs will give them time to test new drivers and make sure everything is working as it should. According to The Verge, even though Microsoft will finalize Windows 8.1 later this month, new machines running the software will be made available in October as well, even though September availability is theoretically possible.
Windows 8.1 is a fairly ambitious update. It will bring back the Start button (but not the Start menu), new tile sizes, improved Start screen customization, Bing desktop support, a new Windows Store, Internet Explorer 11, and more.