Download latest Android KitKat 4.4.2 stock firmware for Acemax K1 S2 TV Box from here, here or here. To flash firmware on this box from SD card read this article. Download previous firmware from here.
Archive： K1 S805+S2 20150505
Image file name: KI-ota-20150505.V0823-s2.zip
Firmware: Android KitKat 4.4.2 stock firmware
Image file size: 453 MB
Platform: Amlogic S805
Build number: KI-user 4.4.2 KOT49H 20150505.V0823 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.