Microsoft just released Windows 8.1, Windows Server 2012 R2 and System Center 2012 R2. You can download Windows 8.1 in the Windows Store or the ISO files for Windows 8.1, Windows Server 2012 R2, System Center 2012 R2 and of course Hyper-V Server 2012 R2 from MSDN or TechNet.
Remember if you were using the RTM Version of Windows 8.1 and Windows Server 2012 R2 which was released some weeks ago, you have to deploy some Update Rollups to get the latest version. The Windows 8.1 and Windows Server 2012 R2 GA build already includes those updates.
And Microsoft also updated a lot of Windows Apps in the Windows Store which should be automatically installed on your Windows 8.1 System