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Windows Phone 8.1 is the third generation of Microsoft's Windows Phone mobile operating system, succeeding Windows Phone 8. Rolled out at Microsoft's Build Conference in San Francisco, California, on April 2, 2014, it was released in final form to Windows Phone developers on April 14, 2014 and reached general availability on August 4, 2014. All Windows Phones running Windows Phone 8 can be upgraded to Windows Phone 8.1, with release dependent on carrier rollout dates.