To help you in updating the email address associated to this account without affecting the other company, I suggest reaching out to our Customer Support. They have the tools to update this information in a secure environment.
You can reach our Customer Support for QuickBooks Online (QBO) by going to the Help icon at the top right of the account. Follow the steps below:
Go to the Help icon to connect to a live support agent.
Provide us with some information about your concern.
Select the Contact Us button and provide some details about this concern, then on the Let's talk button.
You'll now be provided options on how to connect to our Customer Support.