There are a few methods you can use to update your voice mail message.
If you use Microsoft Outlook, click File - Manage Voice Mail (you can get to the same screen by logging on to webmail.smu.edu/owa, click options/see all options, then click phone). From here, click "Call me to play or record the selected greeting..." and enter your phone number. You will receive a call which will allow you to record your greeting.
You can also change your greeting by calling 214-768-8673. Enter your 5-digit extension and your PIN, and then say "personal options" at the voice prompt. Press 2 and you can record your new greeting. The prompts will walk you through these steps when you dial the number.