In order to hard reset a Nokia Lumia 620, you should perform the following steps.
Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
Nokia Lumia 620 device will be brought back to the default configuration of a brand new device.
1. Turn off the phone.
2. Now press & hold the Power and Volume Down buttons. Let go of them when the phone vibrates.
3. Exclamation mark will appear on the screen. When it does, press this button sequence: Volume Up > Volume Down > Power button > Volume Down
4. Your phone will now reset and restart. Voila! Done.