Hard Reset ZTE Nubia X6

First method:

1. First of all, turn off the device by holding the Power key.
2. Then press and hold Volume Up + Power button for a couple of seconds.
3. You should release held keys when you see an Andoid image.
4. In the next step press the Power key to enter Recovery Mode.
5. Choose "wipe data/factory reset" by using Volume buttons to navigate and Power key to confirm.
6. Select "Yes--delete all user data". Use Volume Down button to scroll and Power rocker to select.
7. To restart the phone, select "reboot system now".
8. Success! The hard reset is done!

Second method:

1. Turn on the phone by using the Power button. 
2. After that, go to Settings and then to Backup & Reset.
3. Choose the Factory Data Reset.
4. In the next step tap Reset Phone. 
5. Select Erase Everything if you are sure you want to clear all your data. 
6. Well done! The hard reset has just been performed.

WARNING !
Hard Reset will erase all of your data
All described operations you are doing at your own risk.