Thank you for posting here in the Community. I can provide some information about how you can change the email address of your QuickBooks account.
There are two ways to update your email address. You can either edit the information from your Intuit account or directly from QuickBooks Online.
Since you're unable to change the email address through QuickBooks, please try these steps:
If you no longer have access to the old email address, you can fill out an Email Change Request form. Please attach a copy of your photo ID and make sure the ID is clear, legible, and of the account owner.
For additional reference, you can use the following article to edit user access in QuickBooks: Add, delete, or change user access.
Fill me in if you have additional questions about managing your account in QBO. I'm always here to help. Take care always.