Download Android KitKat 4.4.2 stock firmware for CS918S TV Box from here or from here. Download a newer firmware from here.
Model： CS918S TV Box
Image file size: 353 MB
Platform: Allwinner A31
Build number: mars_ap6210-eng 4.2.2 JDQ39 20140815 test-keys
FW version: v1.2
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.