Download latest Android KitKat 4.4.2 Stock Firmware for ZTE Nubia Z5S NX503A smartphone from here or here. Download another ZTE firmware from here.
Update file name: ZTE Nubia Z5S NX503A 4.4.2 fw
Archive size: 440 MB
Platform: Qualcomm Snapdragon Krait 400
Please do not update firmware unless you know what you are doing, you really want it or it is absolutely necessary. Firmware updates may occasionally have bad results, like bricking your device, temporarily or permanently. The firmware update process is done entirely at your own risk. Before attempting to update the firmware, back up your data.