You can change your PIN whenever you like by going to this page (http://www.neopets.com/pin_prefs.phtml) and following the instructions. You will be asked to enter your current PIN if you want to change it.
To disable your PIN, go to the "Delete and Disable Your PIN" section of the PIN Preferences page and click the box labelled "Delete and disable the use of a PIN with your account." Then, enter your PIN and click "Submit."