It's a free download if you already have Windows 8    



From DailyTech:

Microsoft has made Windows 8.1 available for download starting at 7 a.m. EST today, and it will roll out around the globe over the next 24 hours. The boxed version will be available in retail stores starting tomorrow, as well as brand-new devices that already have the OS pre-installed.

Those with Windows 8 will see various changes in the move to 8.1, such as greater customization. Windows 8.1 brings new background color options as well as different sizing of live tiles.

  Microsoft Rolls Out Windows 8.1