This article describes how to change the voicemail password.
INSTRUCTIONS & INFO
To change your voicemail password, you must know your current voicemail password. Note: If you have forgotten your existing voicemail password, you will not be able to access Voicemail until you reset your voicemail password.
From the home screen, tap Phone Note: This action requires visual voicemail to be active.
Tap the Voicemail icon.
Tap the Menu icon.
Tap Change password, then follow the on-screen instructions to change the voicemail password.