Download Nougat 7.1.2 stock firmware for Minix Neo X39 TV Box from here or here. Download previous firmware from here. Download Rockchip Driver Assistant v4.8 from here and download Rockchip Batch Tool v1.8 from here.
Model: Minix Neo X39 TV Box
Image file size: 904 MB
Platform: Rockchip RK3399 / RK330C
Image name: X39-V003-Android_7.1.2-20200731.img
FW version: 7.1.00
Boot version: 1.15
Caution: Please do not update firmware unless you know what you are doing, you really want it or it is absolutely necessary. Firmware updates may occasionally have bad results, like bricking your device, temporarily or permanently. The firmware update process is done entirely at your own risk. Before attempting to update the firmware, back up your data.