If your phone is stuck in a boot loop or the OTA fails, the official HTC ROM Update Utility (RUU) is the best recovery tool. Steps to Follow:
Try a different USB port (preferably USB 2.0) or reinstall the HTC drivers. Firmware Change Update on HTC One X9
Ensure your HTC One X9 has at least 60% battery to prevent it from shutting down mid-process. If your phone is stuck in a boot
Rename the downloaded firmware ZIP file to 2PS5IMG.zip . Firmware Change Update on HTC One X9
Are you updating to or just for the latest features ? Is your device unlocked or tied to a specific carrier ?