Windows Server 2012 R2 Update is available for download for MSDN and Technet subscribers and it will be generally available through Windows Update starting on April 8, 2014. So what's in this update?
Internet Explorer 11 now includes an “Enterprise Mode.” This mode is available if you disable Enhanced Security Configuration and provides better compatibility for sites designed for Internet Explorer 8, along with tools to manage which web apps use it. You can specify a list of web apps to render dynamically in Enterprise Mode, avoiding compatibility problems with web apps written for older versions of Internet Explorer. For more information, see the Internet Explorer 11 FAQ for IT Pros or TechNet documentation.
- The user interface includes buttons for Search and Power on the Start page. If you install the Desktop Experience feature, other changes introduced with Windows 8.1 Update will also be visible. For details, see What's new in Windows 8.1 Update and Windows RT 8.1 Update. The specific Windows 8.1 features installed when you install Desktop Experience have not changed from those in Windows Server 2012 R2 (see Desktop Experience Overview).
- Active Directory Federation Services (AD FS) has added the capability for an administrator to enable signing in with an alternate login ID that is an attribute of the user object in Active Directory Domain Services (AD DS). This enables customers to adopt Azure Active Directory without modifying on-premises User Principal Names (UPNs). It also allows users to log in to the computer using an Office 365 email address instead of a UPN. This change does not affect the Active Directory schema. For more information, see Configuring Alternate Login ID.
- The update includes all other updates released since Windows Server 2012 R2 was released.
To confirm the exact version of Windows Server 2012 R2 that is installed on a computer, run Msinfo32.exe. If Windows Server 2012 R2 Update is installed, the value reported for Hardware Abstraction Layer will be 6.3.9600.17031.
Microsoft Volume Licensing customers who are planning new or upgrade Windows Server 2012 R2 deployments will be able to get the new OS image through the Volume Licensing Service Center starting on April 14, 2014. We are also working with our server OEM partners to have their offerings updated to the Windows Server 2012 R2 Update image as quickly as possible. We expect various OEMs to roll out systems with Windows Server 2012 R2 Update quickly, but at different times.
Additional note from the Windows Server team blog:
No system APIs have been changed and there are no changes to hardware requirements. Aside from a single reboot, there is no action for you to take if you have turned on automatic update. Plus, you will not need to re-verify nor re-certify your server applications, including System Center. They should “just work” without requiring any additional testing.