On the physical keys of your phone, specifically on the left side, you will notice an "OK" button.

Press and hold the "OK" button for 5 seconds until a prompt appears on your screen that reads "Reset to factory default." Release the physical "OK" button you were holding and proceed to tap the on-screen "OK" button to confirm the action.

Keep in mind it is normal for the phone to take some time to boot back up and restore service.