Download latest Android KitKat 4.4.2 110k4 stock firmware for MK808B Plus from here or from here. To flash firmware on this box via AMLogic USB Burning Tool read this article.
Download previous firmware from here.
Archive name: Transmart MK808B Plus_110k4_20150422
Image: MK808B Plus_109k4_20141222.img
Archive file size: 405 MB
Platform: AMLogic S805
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.