Go to the DahuaWiki and use their Firmware Tool to type in your model number and get a direct download link.
Some sites compile links for various DVRs based on the hardware version (e.g., V4.02.R11), which can be helpful if your device is unbranded but uses Dahua-compatible hardware. Updating the Firmware Download Firmware DVR 8CH DHUA 16MB rar
Once you have downloaded the file (often in a .rar or .zip format), extract it to get the .bin file. You can then update using these methods: Go to the DahuaWiki and use their Firmware
Never unplug the DVR during the update process. It will reboot automatically once finished. Download Firmware DVR 8CH DHUA 16MB rar