In order to hard reset a Sony Ericsson C901, 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 C901 device will be brought back to the default configuration of a brand new device.
1. Switch on the device.
2. Select Menu -> Settings -> General -> Master reset.
3. Follow the on-screen instructions. Voila.