Otherwise , Don't do it because , it will delete all your data of lumia 520 .
Note :- Your all mobile data will be lost.
Steps are given below :
1. Turn off your mobile phone.
2. Then press and hold VOLUME DOWN + CAMERA KEY + POWER KEY simultaneously.
When, mobile get vibrates , then release the POWER KEY, but keep continue pressing VOLUME DOWN + CAMERA KEY, until the phone RESTART. done