Today, Microsoft announced the availability of Windows 8.1, which will come as a free upgrade on October 18th via the Windows Store and retail outlets. If you plan on downloading the update via the Windows Store, you can grab the update on the 17th (at 4AM, PDT) as that time aligns to 12:00 AM in New Zealand on the 18th.
Windows 8.1 Preview has been available to download since June and there have been a few updates and bug fixes released through Windows Update since then.
Windows 8.1 comes with a new Start button on the desktop, different sized tiles, along with many other enhancements.
You can read more about it at the Windows Blog.