Download Android 7.1 stock firmware for X99 TV Box from here, here or here. Download previous firmware from here. Download Rockchip Driver Assistant v4.4 from here and download Rockchip Batch Tool v1.8 from here.
Archive: X99_20180522 (rooted)
Model: X99 TV Box
Image file size: 757 MB
Platform: Rockchip RK3399 / RK330C
Image name: X99 4GB/32GB RK3399 4K TV Box Firmware Update 20180522 / X99_20180522 (rooted).img
FW version: 7.1.01
Boot version: 1.09
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.