The thermostat can be unlocked by pressing the RESET button on the back of the faceplate.  It is a round hole on the bottom right of the thermostat.  Insert the tip of a pencil or ballpoint pen to depress the button located in this hole.  This will reset the thermostat and delete the lock code.  If you do not want to use the lock feature turn off  the dipswitch  for the lock on switch 2 and press the RESET button. 

Note:  If you press the reset button you will lose all other programming.