WARNING! By factory resetting your phone, all of your personal data located on the phone’s internal memory will be deleted, including ringtones, contacts, media files, apps, downloaded content etc. Furthermore all settings will be restored to factory values but data located on the SIM and SD cards won’t be affected.

NOTE: You can check this thread for instructions on how to back up and restore data on the phone.

Factory Reset via Settings:

1. Launch “Settings”
2. Go to “Backup & Reset”
3. If your phone is protected by a lock screen, enter your password/PIN to proceed and tap “Continue”
4. Tap “Reset phone” to confirm

Factory Reset via Android Recovery:

1. Turn the phone Off if necessary
2. Press and hold “Power” and “Volume down” simultaneously
3. At the Boot Mode Selection menu, release the keys
4. Then press “Volume down” repeatedly to navigate the menu and highlight “Recovery”
5. Press “Volume Up” to conform the selection
6. At the “No Command” screen, press and release “Power” and “Volume Up” several times to gain access to Android Recovery
7. Once in the Recovery menu, press “Volume down” repeatedly to navigate it and highlight “Wipe data/factory reset”
8. Press “Power” to select the option
9. Highlight “Userdata only” or “Userdata + Personalized content” and then press “Power”
10. After the process is done, highlight “Reboot system now” and press “Power” one last time to confirm