I need to reset a password from a previous employee on a Partner 18D.

No, whoever administers the voice mail will need to go into administration programming to reset the mailbox or mailbox password.

Depending on the type of voicemail you have, you''ll have to login to the admin mailbox. That's done by pressing your voicemail key or intercom 777. If you have a Partnermail VS the admin mailbox is 99 with the default password of 1234. If you mave a Partner Messaging System the admin mailbox is 0 with a default password of 1234 or 123456. Once your into the admin mailbox press 9 for admin programming. ,3 for greetings, 1 for auto attendant number, 1 for day greeting 2 for night( if your using a night key at the receptionist station then it usus the night greeting) # to go past selector codes, 1 to start recording 3 to accept greeting hang up when done.
As far as the employees voicemail messages, while in the voicemail administration you can reset her password in the "administer mailboxes" prompt which is either 2 or 4 depending on the voicemail type you have. that will reset her password back to 1234.

Changing the Greeting on a Partner Voicemail.

Here. If you dont know the passwords, we are in trouble.

What voicemail do you have currently?? When you dial into voicemail, what does the voicemail lady say? "Welcome to Partner......."??????? Need to know that first.

This should do ya.

http://dstech1.com/changing-time-on-a-partner/

Delete the mailbox for this extension then re-create entire mailbox.

You will have to call in a pro. We can not put the master passwords on the open internet.

Which Partner voicemail do you have? What password do you want to reset? A user or a admin????

To change a voice mail password you would need to contact the service provider and explain the situation and they will reset the code. You may need to visit the provider directly as they may require valid identification.

Go here http://support.avaya.com/elmodocs2/partner_acs/r7/518457701_2.pdf