In order to hard reset a Sony Ericsson P900, you should perform the following steps.
Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
Sony Ericsson P900 device will be brought back to the default configuration of a brand new device.
1. Locate the Wheel. Press and hold it; turn it Up > Down > Down > Down.
2. Phone should now reset and restart, thus completing the factory reset process.
3. Voila, done.