It will be available at retail stores for purchase on new machines. It will also be available as a free upgrade to Windows 8 from the online Windows store.
You can read their full announcement at the link below:
Blogging Windows – Mark your calendars for Windows 8.1!
You may be asking “What about Windows Server 2012R2 and System Center 2012R2?”
Answer: I am glad you asked. They are releasing on the same day! October 18th.
For details on that see Brad Anderson’s announcement here:
Mark Your Calendars for Oct. 18: The R2 Wave is Coming!