I think you'll need to call customer service (800-922-0204) OR *611 fron your handset.
**I did a quick search and found this reply to your question:
Password ResetIf I forget my Voice Mail password, how can I get it reset? There are 3 ways you can reset your Voice Mail password:
If you are already registered for My Verizon, sign In and select My Phone. Then select Reset Voice Mail Password to reset your password.
If you are a Verizon Wireless customer and have not previously registered in My Verizon, please Register. After you have completed the requested information you will be able to perform your Voice Mail password reset online.
From your wireless handset: press *611, then SEND (airtime free) or call (800) 922-0204 (toll free) from any phone to reach the easy-to-use automated customer service menu. Select option 2 to reset your Voice Mail password, and then press 1 when prompted for the password reset menu. You will be asked to enter your 5-digit billing zip code. Follow the prompts for security verification and resetting the password.