Download latest Android KitKat 4.4.2 stock firmware for Nagrace HPH NT-V6 TV Box from here or from here. You can download previous firmware version from here.
Firmware: TRN6A_HPH-FO-N6_RK3288_4.4.2_EN_US_FT0_V1.4.4_150129
Model: HPH NT-V6
Image file size: 439 MB
Platform: Rockchip RK3288