I am going to include a link to the administrator guide for you to download (its in pdf format). However here is what I found in the admin guide:
To change a password:
1. Access the voice mail administrator's mailbox and menu (see page 53).
2. Say "Mailbox Maintenance" or press to access the Mailbox/Group List Maintenance
3. Say or enter the mailbox or extension ID to be accessed. You hear the Personal Options
4. Say "Password" or press .
5. If you want a password, enter a new password using digits 0-9 (up to 12 digits) and
press when you are done.Voice mail plays back the password.
If you do not want to use a password, say "Skip" or press . Voice mail announces
that you have chosen not to have a password.
6. Say "Accept" or press to accept the password as entered, or say "Re-enter" or press
to erase and re-enter the password.
7. Hang up or press to return to the previous menu.
If this is not the right section then maybe one of these admin guides will help:
Hope it all goes well for you