Before downloading anything, note your existing version. On your ZTE box:
Fixing freezing, lag, or boot loop issues. Zte Zxv10 B760hs3 Firmware Download
Download the SP Flash Tool or the specific ZTE Smart SW Tool compatible with MT8639 processors. Before downloading anything, note your existing version
If your device is functioning but you want to apply a signed update package, you can use the built-in Android Recovery. Before downloading anything