Download latest Android KitKat 4.4.2 stock firmware for MXIII-G Android TV Box from here or from here.
BTW, you can now buy MXIII-G Android TV Box from gearbest.com for only $63.99 with free shipping.
Model： MXIII-G Android TV Box
Image file size: 506 MB
Platform: Amlogic S812
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.