If you go to http://www.windows.com/buy, the site will detect your current operating system and provide you with the steps to upgrade. If already running Windows 8.0, you can just start the free upgrade process.
Check out this video showing Windows 8.1 running on all sorts of devices.
My Windows 8.1
To get the full details of the release, have a read of Brandon LeBlanc post on the Windows blog site:
To learn more about Windows 8.1, have a look at the Meet Windows site and watch the video:
Meet the New Windows 8.1
For more Windows 8.1 information have a look at these related posts:
Windows for your Business Blog: Windows 8.1: One Vision, One Platform for everything you do at work
Windows App Builder Blog: Now ready for you: Windows 8.1 and the Windows Store
Windows Experience Blog: Facebook for Windows 8.1 now available