How to Hard Reset NOKIA Lumia 520: Windows Phone 8 Method
1. First of all, power off the phone by using the Power button.
2. Press and hold the Power key for a short moment.
3. Release the Power button as soon as phone vibrates and start holding Volume Down key.
4. When the exclamtion mark appears on the screen release the key.
5. Now, press the following buttons in the sequence they are written here: Volume Up - > Volume Down - > Power - > Volume Down.
6. Congratulations! The hard rest has just been performed.
Factory Reset from within the Phone
1. Turn on the phone if the phone is off.
2. Go to the Main Menu, find and select Windows Phone Settings then, About.
3. Afterwards tap Reset your phone and confirm it.
Success! The device is ready to use.
Hard Reset Lumia 520 : Windows Phone 7 Method
1. Power off your phone.
2. Press and hold Volume Down + Camera key + Power .
3. When the phone vibrates, then release only Power button.
4. When your phone reboots, then release the reset of the keys.
Note: You can always use either of the methods illustrated above to Hard Reset your Nokia Lumia 520 in the event that you are unsure of the Windows Mobile version on the phone.