How to Factory Reset Nokia Lumia 520 625 720 920 820 / Windows 8 Mobiles
* Before perform factory restore, you have to Backup all your data. Factory restore will erase all the data inside your mobile phone including Contact details.
Method .1(Hard Reset)
On the start screen, Go to Settings > about > reset your phone.
If your Mobile Phone keeps restarting continuously and not even allowing you to go inside, then you can do factory reset using the below Method.
1. Disconnect your Mobile phone from the charger.
2. Press and hold the volume down key and connect the charger. An exclamation mark (!) will shown on the screen.
3. Press the following keys in order: volume up -> volume down -> power -> volume down.
Your phone resets and boots up automatically. This will take several minutes.
Once your phone is restarted, This will show the factory default boot settings.
If your data is already backed up in windows online storage, then You can restore the backup data.