Let's ensure you'll be able to update the email address associated on your account.
To start, I would like to know what specific error message you're getting when trying to update email. This way, I can gather enough details and gives you the right amount of information.
As an initial step, you can open your QuickBooks Online (QBO) account using a private browser, then update the email address from there. To save you time, you can use either of these keyboard shortcuts: