If you don't care for data loss, follow this Procedure:
"Master reset with hardware keys
A master reset restores the original factory settings and may delete your personal data on the internal storage, such as downloads, ringtones, images, apps, contacts, and Visual Voicemail. It does not delete data stored on the SIM card or SD card. If the device menus are frozen or unresponsive, you can master reset using hardware keys.
Back up data on the internal memory.
With the 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.
Wait as the device resets."
for more info go to https://support.t-mobile.com/docs/DOC-27502#steps2