The highly anticipated and well-reviewed “Mango” update to Microsoft’s Windows Phone operating system is expected to be available for some devices in the U.S. as early as this week — beginning a new bid by Microsoft to reverse its fortunes against the likes of Apple and Google in the mobile phone market.

Existing users of Windows Phones on AT&T’s network will have access to the Mango update (a.k.a. Windows Phone 7.5) on Tuesday, according to a report by the WPCentral news site. That’s consistent with the timing of “in the next week or two” laid out by the Windows Phone team in a post of its own last week.

Mango adds more than 500 new features, including upgraded Live Tiles on the home screen, better integration with third-party apps and new communications features. (See the Microsoft video below for an overview.)

No word yet on specific timing for the Mango update for Sprint, Verizon and T-Mobile customers, but devices running Mango have been spotted in stores in New York and Las Vegas. Carriers including AT&T are also planning to come out with new devices running the Windows Phone update this fall.

The imminent rollout of Mango is good news for Microsoft, which experienced extended delays as carriers tested earlier minor updates to Windows Phone software. Microsoft’s market share has fallen into the single digits in the U.S., as Apple’s iPhone and Google’s Android have taken hold. However, one recent study showed that Microsoft has succeeded in getting more than 40 percent of smartphone buyers to at least consider getting a Windows Phone.

Long term, Microsoft is counting on a partnership with mobile phone giant Nokia to boost Windows Phone’s prospects.

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