In order to hard reset a Nokia Lumia 810, 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 810 device will be brought back to the default configuration of a brand new device.
1. Turn off the device.
2. Press & hold the Power & Volume Down buttons. Let go of them when the phone vibrates.
3. Exclamation mark will appear on-screen. When it does, press these buttons in exacly this order: Volume Up > Volume Down > Power button > Volume Down
4. Wait for the phone to reset and restart. Done.