Download the firmware from GMMZ. The site is in Thai language, but understandable after enabling auto-translation in Google.

After unzipping, copy the the level with the file “factory_update_param.aml” to a USB stick. Modify the line with the path to the firmware image and add the folder name after “/udisk/”, for example:

--update_package=/udisk/22022019/u2x-update-20190222.zip

Disconnect power, then push and keep the reset button inside of the AV connector, and reapply power. The firmware update will execute without further interaction.