Go to the DahuaWiki and use their Firmware Tool to type in your model number and get a direct download link.
If your device is connected to the internet, you may be able to enable Cloud Update via the mobile app iDMSS/gDMSS or the DVR menu to skip manual file handling. Download Firmware DVR 8CH DHUA 16MB rar
Copy the .bin file to a formatted USB stick, insert it into the DVR, and follow the prompt for System Upgrade . Go to the DahuaWiki and use their Firmware