A firmware update was recently released for the NVR6XX-4KS2 series devices.
Information, instructions, and download links can be found Here
Firmware Search Tool/DVR
From Dahua Wiki
DVR Firmware Search Tool
If you are unable to locate your device's firmware, you can update your device with the DVR Firmware Toolkit.
Method for Updating Firmware Links
Use these links to use a method for updating firmware: