A firmware update was recently released for the NVR6XX-4KS2 series devices.
Information, instructions, and download links can be found Here
From Dahua Wiki
What if network upgrade failed?
- Log in again to check if upgrade is successful, because sometimes the progress bar doesn’t show feedback in time due to the network environment.
- Check if the network environment is normal, and there is disconnection or power failure during upgrade.
- Check if the program and the device are matched.
- Re-upgrade through 3800 port of config tool (3800 needs to upgrade matched program).