If you want newer firmware features (like VoLTE or better power management), your only options are:
: Firmware versions can be upgraded but generally not downgraded unless the device is in S-OFF mode. Firmware Change Update on HTC One X9
Copy the file to the root directory of a microSD card (not inside any folder). Insert Card: Put the SD card into your HTC One X9. If you want newer firmware features (like VoLTE
. If the connection broke now, the firmware would be corrupted. After ten tense minutes, the HTC One X9 vibrated and rebooted. The familiar HTC logo appeared, followed by the "Android is starting" screen. Firmware Change Update on HTC One X9
The HTC One X9 vibrated once, as if to say: Don’t thank me. Thank the firmware.