via The Verge:
Microsoft finalized its upcoming Windows 8.1 Update recently, but the company appears to be sharing it a little early today. While the software maker is expected to release the update officially in April, Microsoft has only detailed a few of the features in the update and has not yet provided an official release date. Links to download a final version of the Windows 8.1 Update, thanks to Microsoft’s Windows Update service, have been discovered. A series of patches are required to obtain the full update, but once installed the new desktop-friendly features are enabled.The update can be downloaded via a registry change, or through direct links, but we recommend waiting for Microsoft to officially release it through the normal Windows Update channels in April.
You can find the direct download links from Microsoft in this forum post.