In order to hard reset a Nokia Lumia 900, 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 900 device will be brought back to the default configuration of a brand new device.
1. Turn off the phone.
2. Press'n'hold the Volume Up and Camera and Power buttons.
3. Let go of the Power button when the device vibrates.
4. After a while, phone will reset and restart. Voila! Done.