how to reset passcode

To reset the passcode (if you know the current passcode):

General > Passcode lock > change passcode

That depends (and I am assuming you are talking about the lock passcode): If you have forgotten it, you will need to reset the device to factory settings via itunes. If you simply wnat to change it (and you know the current passcode), Settings app > General > Passcode Lock > Change Passcode

what if you dont know the password?

You will need to restore it as a new devcie.