What firmware version does it have currently?
The newest firmware (3.4.80 and I think the 3.4.62 version) have a feature where they prohibit downgrading, not just in the running firmware but the bootloader as well.
It looks like it may take the same firmware as the 78xx series - the third section down here :
海康威视是以视频为核心的物联网解决方案和数据运营服务提供商,面向全球提供安防、可视化管理与大数据服务。
In which case, if it doesn't have the 'unrollback' (sic) feature, as a complete long shot you may be able to install the very old 3.0.8 firmware from here, select English, and web GUI update through the versions, keeping the EN menus.
http://www.hikvisioneurope.com/portal/index.php?dir=Product Firmware/Back-ends/Recorder/NVR/DS-7600NI-E1(E2) 7700NI-E4/
*edit* Another option might be to use the Batch Config Tool - number 16 here -
http://www.hikvisioneurope.com/portal/index.php?dir=Software/00 Software Tool Package/
and ignore the fact that the native menus are in Chinese.
Though I haven't actually tried that on NVRs - it works pretty well on cameras, once you've found the hidden 'settings wheel'.