The Windows 8.1 Update is available on now for MSDN subscribers, and will begin to roll out to Windows 8.1 and Windows RT 8.1 customers via Windows Update on April 8. The majority of users will receive the update automatically. If you’re still on Windows 8, you can get the Windows 8.1 Update via the Windows Store on April 8 as well.
Cloud and Datacenter Management Blog 