This has PDF file that will give instructions.
Also this link has more instructions:
To Change the Name on a Mailbox:
1. Log into system administration
2. Select the softkey under "MBOX"
3. Select the softkey under "Chng"
4. Enter the mailbox number (extension. you want to change the name on
5. The display shows "Reset Password"
6. Select the softkey under "Next", keep hitting "Next" until the display shows the name
7. Select the softkey under "Chng" to enter a new name
8. Enter the name using the dial pad. Enter last name first, press "# #" to enter a comma and then enter the first name. When spelling the name, press "#" to move to the next character
9. Select the softkey under "Next" to save the change
10. Hit the release button