Because this board is used by multiple manufacturers, there isn't one single "official" website. You generally have three reliable options:
Press and hold the on the TV cabinet (not the remote). While holding the button, plug the power cord back in. hktrt2843p639 firmware link
Place the firmware file (usually named allupgrade_v2843p639.bin or similar) into the root directory of the USB. Do not put it in a folder. The Flashing Process: Unplug the TV from the wall. Insert the USB drive into the USB 1 port. Because this board is used by multiple manufacturers,
Once you have located and downloaded the correct file, follow these steps to flash your HKTRT2843P639 board: Place the firmware file (usually named allupgrade_v2843p639
Do you have the from the sticker on the back of the screen so I can help you find the exact match?