This site has the most extensive collection of TX9 Pro firmware, though you may need a browser translator to navigate it. How to Install the Firmware
Flashing new firmware will wipe everything on the device. tx9 pro android tv box firmware link
This is the most comprehensive archive. Search for "TX9 Pro" to find various versions, including the stock Android 7.1 Marshmallow or newer 9.0 builds. This site has the most extensive collection of