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.
Log into the DVR’s IP address through a browser and navigate to Maintenance > Manager > Update . Important Safety Tips
Never unplug the DVR during the update process. It will reboot automatically once finished. Download Firmware DVR 8CH DHUA 16MB rar
Go to the DahuaWiki and use their Firmware Tool to type in your model number and get a direct download link.
Use the Dahua ConfigTool on a PC to find the device on your network and push the update remotely. If your device is connected to the internet,
A "16MB" firmware file typically refers to the size of the flash memory on the DVR's motherboard, a common spec for many 8-channel units using HiSilicon SoCs. How to Find Your Firmware
Check the sticker on the bottom of the device or the System Info > Version menu in your DVR settings. It will reboot automatically once finished
Do you have the from the sticker or the system menu to help narrow down the exact file? Firmware Update via USB Drive - Dahua Wiki