Download Android KitKat 4.4 stock firmware for M8S+ TV Box from here or from here. To flash firmware on this box from PC via AMLogic USB Burning Tool read this article. Download Android Lollipop 5.1 stock firmware for M8S+ TV Box from here.
Model: M8S Plus TV Box
Image file size: 661 MB
Platform: Amlogic S812
Update file name: M8S+_13052015.img
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.