Microsoft have released Windows 8.1 Update (http://support.microsoft.com/kb/2919355) for Windows 8.1, 8.1 RT and Server 2012 R2.
This release is considered a new servicing/support baseline which starts with May's patch Tuesday. Anyone who has Windows 8.1 must install the Windows 8.1 Update in the next 30 days or they will not receive security updates.
Users of Windows 8 and Server 2012 are not affected.
Further information on the Windows 8.1 Update can be found at:
Downloads of the 32 and 64 bit updates are available on the UIS ftp server
Please note, this is a major update and will take a while to install.