Try doing it through the Optus app. I had problems changing my billing email address and the only thing that worked was through the app, not even Optus staff could fix the problem.
Your contact email for say, My Account, or your primary Optus username/email.
Only customer care can make this change. There's some info here
Can I change my primary Optus email?
Yes, you can ask request to change your primary Optus email or ask us to add up to 14 secondary email address to your account. To change your primary email or to add a secondary email address, send our team a message.