Download latest Android 4.4.2 KitKat stock firmware for HiSilicon Hi3798M TV Boxes from here or here. To upgrade follow this tutorial.
Archive name: HiSilicon Hi3798M update1201-android
Archive file size: 231 MB
Support: HiSilicon Hi3798M
Image file: update.zip
Build number: Hi3798MV100-eng 4.4.2 KOT49H eng.zhaohr.20141118.185949 test-keys