Microsoft has finally announced that Windows 8.1 will be available on the Windows Store for free for existing Windows 8 users at 7AM ET on October 17th. So mark your calendars! On October 18th, the boxed copies and new Windows 8.1 devices will be released. Windows 8.1 RTM will be made available to PC manufacturers before August 23rd.
For those who don't know, Windows 8.1 brings many improvements in areas like personalization, Internet Explorer 11, search, built-in apps including a few new ones, an improved Windows Store experience, deep SkyDrive integration and the Start Button. 
Source: Windows Blog