In the Microsoft Build Conference today, Microsoft has officially unveiled the features of Windows 8.1 Preview and announced the availability of Windows 8.1 Preview which is available for download from Microsoft Download Centre.
The Windows 8.1 Preview is expected to include good number of features like the addition of the start menu, improved multi-tasking etc. The Windows 8.1 update is expected to be free for the existing windows 8 users.
The Windows 8.1 Preview can be downloaded from the Microsoft Download Centre. (Official link)