Hard Reset NOKIA Lumia 820

First Method - By Hardware Keys
At the very beginning turn off the phone by using the Power button.
In the next step hold down the Power key until the device vibrates.
Then press and hold the Volume Down button until you see the exclamation point on your screen.
Now, you can release all keys.
Press the following sequence of buttons: Volume Up - > Volume Down-> Power - > Volume Down.
Success! The hard rest has just been performed.

Second method: By Mobile Settings

Make sure that your phone is switched on.
From Main Menu find and select Windows Phone Settings.
After that tap About.
To confirm the whole operation tap Reset your phone.
Well done!