Nokia C6-00 - How do I restore factory settings on my Nokia S60 phone?

Select Menu > Settings > Phone > Phone mgmt. > Factory settings.

You can reset some of the settings to their original values. To do this, you need the lock code, which is 12345 by default unless you have changed it before.

After resetting, the device may take a longer time to power on. Documents and files are unaffected.