Hard Reset ZTE Blade S6

First method:

1. At the beginning, turn off the phone by holding the Power button.
2. Then press and hold Power + Volume Up button.
3. When you see Recovery Mode menu release held buttons.
4. Press Power key to enter Recovery Mode.
5. Afterward from the menu choose "Wipe Data/Factory Reset" by using Volume Down to scroll and Volume Up to select options.
6. Confirm the whole operation by tappinng "yes".
7. Success! The hard reset has just been performed

Second method:

1. Turn on the phone by using the Power button. 
2. After pressing the Home button go to Settings and then to Backup & reset.
3. Choose the Factory Data Reset.
4. Tap Reset Phone. 
5. Select Erase Everything if you are sure you want to clear all your data. 
6. Well done!

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