You can change your password or update your information (address, phone number, email address...) with Verizon by logging into the Verizon website, clicking on Manage My Profile and then changing your information as needed.
Keep in mind that when you add or change a phone number that is tied to a method in which you receive a passcode, you will also need to update your passcode device accordingly. (ie. If you add a new device that you wish to receive a text message, you will need to add the number to your profile and then go to Passcode Devices and add the device there as well. Simply adding a new number does NOT mean that it will automatically receive a passcode.)
Whenever you change this information (especially your passcode) you will need to resign the Subscriber Agreement. If this is not done, you will not be able to send controlled substances. Always check your Subscriber Agreement and Certificate Status before exiting Verizon.