Go to support.avaya.com and create a login account. Then search for PARTNER Messaging 7.0 System Administration Software, or whatever release you have (R1, R6, or R7)
The Partner Messaging's default IP address is 10.0.0.42, so set your PC to a static address of 10.0.0.43 and use a cross over cable to connect. If you want to put it on your network, you need to change the Messaging's address, which will require either a modem or a serial cable.
Once you are connected, go to the Auto Attendant, click on the button for the greeting, and a player pops up. From there you can listen, save, or upload a file from your PC to replace the current one.
Just read the ReadMe file included in the download.