You'll probably need to contact Optus I suspect to change your phone number - its a pretty key piece of security.
Actually the option is right there. Just select PROFILE in MY ACCOUNT and you can update email, phone, address, etc.
Do you mean not changing your actual phone number but viewing details of one of your mobile numbers on the dashboard instead of the account number?
I don't think this is possible, you just need to scroll down to the data usage area and click on the desired mobile number on there to view details.