Windows 8.1 (codenamed Blue) is an upgrade for Windows 8, a version of Windows NT, a computer operating system released by Microsoft. First unveiled and released as a public beta in June 2013, it was released to manufacturing on August 27, 2013, and reached general availability on October 17, 2013, almost a year after the retail release of its predecessor. Windows 8.1 is available free of charge for retail copies of Windows 8 and Windows RT users via the Windows Store.