Download latest Android Lollipop 5.1.1 stock firmware for M8S TV Box from here or here. Download previous firmware from here. Download latest Android KitKat 4.4.2 stock firmware for M8S TV Box from here.
Model: M8S TV Box
Image file size: 535 MB
Platform: Amlogic S812
Build number: Android/n200C/n200C:5.1.1/LMY47V/20160721:user/test-keys
Inside archive: firmware + recovery.img
Download this zip file it will have all you need and copy it to an sd card, then pop your sd card into your box put a pin in the reset hole and wait untill the TEAMWIN logo appears. After that click install and click on the zip file. click install and wait… then click reboot box and you should now have android 5.1.1 on your m8s ap3660 box!
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.