Download latest Android KitKat 4.4.2 stock firmware for Acemax KII TV Box from here, here or here. To flash firmware on this box from SD card read this article.
Next, you can download Android Lollipop 5.1.1 beta firmware for Acemax KII TV Box from here.
Archive： K2 20150506
Image file name: KII-ota-20150506.V0823.zip
Firmware: Android KitKat 4.4.2 stock firmware
Image file size: 442 MB
Platform: Amlogic S812
Build number: KII-user 4.4.2 KOT49H 20150506.V0823 test-keys
How to update firmware
1.Copy all files listed below to tf card root directory: u-boot.bin, KII-ota-20150506.V0823, factory_update_param.aml, recovery
2.power on the KII box and insert tf card into box.
3. go to SETTINGS-SYSTEM UPDATE–SELECT–Choose Wipe Data, Wipe Media—Update
4. Updating Progress, DON’T POWER OFF!
5. After it’s finished, the box will restart.
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.