duga632000 , Factory/master reset, you will loose all stored data on the device. Info. form link below.
Back up data on the internal memory.
From, device turned off, press and hold the volume up and power buttons until the ZTE logo screen appears, then release.
Press the volume down button to highlight wipe data/factory reset.
Press the power button to select.
Press the volume down button repeatedly until Yes (delete all user data is highlighted).
Press the power button to select.
Now the device should reset, may have to wait a bit.
Links below give more detail in performing resets. Good luck.
I hope this helped you out, if so let me know by pressing the helpful button.
http://www.hardreset.info/devices/zte/?p=1
https://support.t-mobile.com/docs/DOC-27502