如何将Nokia Lumia 520恢复出厂设置/强制重启

i have a challenge with my Lumia 520 i cant see the apps the screen is fine its difficult to format and everything in the phone is ok just that i cant use the phone its been given me issues and i have taken it for repairs they confirmed it needs to be flashed but i dont know what to flash it with as all the technicians i took it too couldn't flash the phone