Check the sticker on the bottom of the device or the System Info > Version menu in your DVR settings.

Log into the DVR’s IP address through a browser and navigate to Maintenance > Manager > Update . Important Safety Tips

Go to the DahuaWiki and use their Firmware Tool to type in your model number and get a direct download link.

Never unplug the DVR during the update process. It will reboot automatically once finished.

Installing the wrong firmware can "brick" your device, making it unusable.