Web Portal:
You will need to login to Avaya Central Office web portal at: https://login.cloudoffice.avaya.com/
Once you are logged in, select Voicemail
Then you'll want to make sure Settings is selected on the menu near the top. Then expand the options under Phone by clicking the down-pointing carrot.
Then scroll down until you see the Voicemail settings options and select Voicemail PIN
Enter your new voicemail PIN twice and click Save
That's it, now you are done!