If your phone is on, power it off.
Hold down the Power button and Volume down button. Instead of booting up normally, you'll see "Start" in an arrow pointed at the power button. Here you use the Volume buttons to scroll and the Power button to select an option
Press the Volume down button until you've highlighted Recovery mode.
Press the Power Button to start recovery mode.
When your phone reloads, you should see a screen that says "No command" with an android robot in distress. Press and hold the power button, then press the volume up button once to load up the reboot menu.
Using the Volume buttons to scroll through the menu, highlight Wipe data/factory reset.
Press the Power button to select.
Highlight and select Yes to confirm the reset.
Once the reset is complete, you will be bounced back to the same recovery mode menu. Press the Power button to select Reboot system now.
Your phone will power on and be reset to its factory settings. The next time you turn the phone on, you will go through the initial setup process as if it were a brand new phone. Once set up, you can go about restoring your data from wherever you had it backed up.