The official firmware files vary by the manufacturer of your specific device: Official Center: Firmware for various Tanix models, including the , can be found at the Tanix Firmware Centre .
Before flashing, you must verify your device's internals to avoid "bricking" the box: tx9 pro android tv box firmware link
Once you have located a link and downloaded the .img file, you generally have two ways to flash it: The official firmware files vary by the manufacturer
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. tx9 pro android tv box firmware link