If you can tell me the or the Hardware ID found in your DVR's "Version" menu, I can help you find the exact download link for your specific board version. Firmware Update - Dahua Wiki
Open the DVR casing and look for a white sticker or printed text on the green PCB. It should say AVR8283 followed by a version number (e.g., V1.01 or V2.0). Download Firmware DVR DHUA 7004AHD (AVR8283 rar
Record your network settings and camera names before updating, as they may be reset. If you can tell me the or the
A common source for V4.02 generic AHD DVR firmware. Download Firmware DVR DHUA 7004AHD (AVR8283 rar
Use a UPS (Uninterruptible Power Supply) if possible to prevent power cuts during the flash.