If you are using the Windows Server Technical Preview or Hyper-V Server Technical Preview you probably noticed that it expires 15.04.2015. Few days ago Microsoft released the patch to extend the activation period until October 2015. It should do until the next publicly available release of Windows Server build. You can download the patch from Microsoft Download Center.
After installing the patch it will take couple of reboots until is finally installed.
If you are patching VM it should go without any problems, but if you are patching a physical machine it may require that you reactivate your Windows. Simply run: Slmgr /ato
After successful patching you should see the following: