In order to hard reset a Sony Ericsson K550, 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 K550 device will be brought back to the default configuration of a brand new device.
1. Power up the phone.
2. Go to Menu > Settings > General > Master reset.
3. Now select Delete all user installed applications > Delete.
4. Lastly, tap in the phone's lock code and select Done.