If you have an MSDN Subscription it should be available to you now.
Windows 8.1 Preview and Windows RT 8.1 Preview are prerelease versions of Windows 8.1. They build on Windows 8 and bring you more ways to stay productive, have fun, and do all the things you need and want to do on your PC. Windows 8.1 Preview and Windows RT 8.1 Preview include enhancements in personalization, search, apps, the Windows Store, and cloud connectivity, and have the security and reliability features you expect from Windows. They’re fast, and made to work on a variety of form factors—especially the new generation of touch devices.
Here’s your chance to be one of the first to try out the preview. We’ll use your feedback to make the final product even better.
For detailed product and feature information, download the Windows 8.1 Preview Product Guide.