Download latest Android KitKat 4.4.4 stock firmware for Transpeed CX939 TV Stick from here or from here.
Model： RK3128 TV Box
Image file size: 475 MB
Platform: Rockchip RK3128
Model number: rk312x
Boot version: 2.20
FW version: 4.4.04
Build number: rk312x-eng 4.4.4 KTU84Q eng.renj.20150625.100918 test-keys
SDK version: RK30_ANDROID4.4.4-SDK-v1.00.00
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.