The safest and easiest way to update your device is through the Over-The-Air (OTA) Scroll down and tap About Phone System Updates The phone will automatically check for available firmware. If an update is found, tap Download and Install Once complete, tap Restart Now 🛠️ Manual Firmware Flashing
If an OTA update fails, try restarting the phone or clearing the cache of the "System Update" app. What is the current status of your device? malfunctioning or bricked
Sites like HardReset.info maintain databases for ZTE stock firmware.