Microsoft has released Windows 8.1 Update. You can install it via 'Windows Update' or manually download the update by clicking on the below given links.
It is recommended to install Windows 8.1 Update via Windows Update but if you can't wait then make sure to download and install all the below linked KB files which contain necessary files to successfully deploy Windows 8.1 Update. If your automatic update option is enabled or you are sure that your copy of Windows 8.1 is up to date then KB2919355 is the only update you need to install.
Main update file for up to date systems:
Windows Server 2012 R2 Update (KB2919355)
Necessary files to successfully deploy Windows 8.1 Update on x86 system:
Download links via Winbeta.