Download Android Marshmallow 6.0 stock firmware for M8S Pro TV Box from here, here or from here. Download previous firmware from here. Download Rockchip Driver Assistant v4.4 from here and download Rockchip Batch Tool v1.8 from here.
Archive: M8S PRO RK3229 20170720
Model: M8S Pro TV Box with 2GB RAM + 8 GB ROM
Image file size: 476 MB
Platform: Rockchip RK3229 / RK322A
Image name: M8S PRO RK3229 20170720.img
SDK version: RK30_ANDROID6.0.1-SDK-v1.00.00
FW version: 6.0.01
Boot version: 2.32
Model number: M8SPRO_6.0
Build number: rk322x_box-userdebug 6.0.1 MXC89L user.foxluo.20170720.162547 test-keys
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.