Microsoft CEO Steve Ballmer holds two Windows phones as he speaks during his keynote address at the Microsoft Build Conference in San Francisco. MICROSOFT has debuted a "re-blended" version of the overhauled Windows 8 operating system released late last year. It also previewed new office apps for its Windows Metro interface, but said they would not be released until next year. Windows 8.1 incorporated feedback from users and developers, and came with the promise that the US software giant was speeding up its release cycle to adapt to the dizzying pace of innovation in consumer technology. "We pushed boldly in Windows 8 and got lots of feedback," Microsoft chief executive Steven Ballmer said while kicking off the company's BUILD developers conference in San Francisco. "Users said 'Why don't you go refine the blend?," he said. We will show you a refined blend of our desktop experience and our modern experience. Cheers burst from the audience when Ballmer assured them that changes in Windows 8.1 included a return of the "Start" button icon on screens that provided shortcuts to commands and applications. Microsoft has made a preview version of Windows 8.1 available for developers online at preview.windows.com and said this was just the beginning of a shift to "rapid release" cycles for software. "Rapid release cadences are absolutely essential to what we are doing," Ballmer said. It is about the transformation that we are going through as a company to move at an absolutely rapid release cycle; our transformation from a software company to a company building software-powered devices and services. Windows 8.1 was remaining true to the vision of an operating system tuned for touch-screen controls and multi-gadget lifestyles increasingly revolving around tablets and smartphones, according to Microsoft. "When we rolled out Windows we talked about touch, touch, touch," Ballmer said, noting that when people went to stores there was a dearth of Windows-powered touch computers. He said there would be a "proliferation" of small Windows tablets released in the coming months. Microsoft used the keynote presentation before BUILD's six thousand attendees to showcase Windows-powered devices ranging from Nokia Lumia smartphones to Lenovo and Acer devices as well as Microsoft's own Surface Pro tablet. "You will see an outpouring of new devices that are notebook computers in every respect yet have touch fully integrated and usable," Ballmer said. Microsoft also announced that it was opening its Bing internet search engine to developers so they could harness its capabilities to power features inside applications.