for the latest official build numbers and security patch history for the Z999. ZTE Opensource repository
: You can check for official firmware updates directly on the device by going to Settings > AT&T Software Update > Check for Updates SD Card Manual Updates zte axon m z999 firmware
Most official builds are based on Android 7.1.2 (Nougat) with custom ZTE overlays. for the latest official build numbers and security
Use the volume keys to highlight and press the Power key to confirm. a USB cable
Flashing this device requires because ZTE locked down the traditional fastboot flash commands. You will need a Windows PC, a USB cable, and the QPST (Qualcomm Flash Tool) .
Here is detailed information regarding the firmware.