How to format Nokia Nseries phone

*#7370# (*#RES0#)- Reset your phone to the default factory settings. Usually you will need this code when you want to reset your phone back to the same condition when you got it for the first time. When the phone ask for security code, the default one is 12345.
Warning: Before you do reset the phone, make sure that you don't have important data. Your data on the phone memory will be lost once you reset it.

*#7780# (*#rst0*) - This will soft reset your phone. It is different from *#7370# because all data, like contacts, calendar and notes, are not removed. It will only reset the phone settings, such as profiles, themes and shortcuts. The default lock code is 12345.

There is another key combination which is used to hard reset the phone (similar to #*7370). It is useful in some cases, for example when the phone cannot reboot. The trick is by switching off the phone then press and hold Call key (Green key) + * key + 3 key + power button for a couple of seconds. Note that you have to press and hold the four keys at the same time; otherwise it won’t reset the phone. This key combination doesn't work on some S60 phones.