Download Android 7.1 stock firmware for M96X Plus TV Box from here and here. Download previous firmware from here. Download Amlogic USB Burning Tool v2.1.6 from here or download Amlogic Burn_Card_Maker v2.0.2 from here.
Archive: M96X Plus 20180602
Model: M96X Plus 2GB/16GB TV Box
Image file size: 692 MB
Platform: Amlogic S912
Build number: s912011-userdebug 7.1.2 NHG47L 20180602 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.