Microsoft has finally released the Windows Phone 8.1 and is now available for download for registered developers.

Early this month, the software giant has introduced Windows Phone 8.1 during the developer conference held in San Francisco, Calif. This software update will include Cortana-a virtual assistant powered by Bing similar to Google Now and Apple's Siri. However, according to The Verge, this feature is only available in the United States at the moment.

Other features included in the Windows Phone 8.1 are the Action Center, which looks like Tiles that will automatically display new activities and notifications; Senses, that allow users to manage data, space, and battery life; and Wi-Fi Sense, that automatically detects trusted Wi-Fi hotspots and allow users to share passwords and usernames to other devices. It has also incorporated the latest Internet Explorer 11 as its default web browser and a new YouTube web player that can support HTML5 videos and closed captions.

Microsoft has also included business apps and services such as simplified device enrollment, a built-in mobile device management client, S/MIME for encrypted mail and VPN support.

The release of the OS update comes ahead of the initial announcement. During its introduction, Microsoft said that Windows Phone 8.1 will not be released until the next coming months. However, the update is already pre-installed on new phones released April including Lumia 630 and 635, as well as Lumia 930, The Next Web reports.

Cortana will be limited to U.S users for now but will be available in U.K and China on the second half of this year. Other countries will have to wait until next year.

Those interested to download must be aware of the limitations. Upon downloading, any warranty from the mobile operator and OEM will be automatically void. There is also no way to revert to the previous OS after migrating to the new OS.