Because this is a hardware part number rather than a software release name, finding the "firmware" requires understanding the relationship between the board and the printer model it powers.
Keep holding until the LED indicator starts flashing (indicating the update has started). Completion: Wait until the TV reboots automatically. Do not disconnect power during this time to avoid "bricking" the board. 4. Troubleshooting & Rollback Boot Loop: tpmt5522pc821 firmware