Download latest Android KitKat 4.4.2 stock firmware for CS918S TV Box from here or from here. Download previous firmware from here.
Firmware image file: CSQ-A31-500W-中文-20150327-2GB16GBpub.img
Model： CS918S TV Box
Image file size: 462 MB
Platform: Allwinner A31
Build number: mars_A31zh-eng 4.4.2 KOT49H 20150327 test-keys
FW version: v4.4.2
Pixels: 1280 x 800
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.