If I edit a parent customer e.g email address - how do I make it pull the new email address through to the sub-matters under that parent customer?

Hello, @des10.

I can help walk you through the steps on how to modify the customer email address in your account. 

Are you referring that the new email address will flow to the old transactions in QuickBooks Online (QBO)? If so, changing the parent customer email address in QBO will take effect only on the new or upcoming transactions in your account. 

Here's how to update email address: 

1. Select Sales, then choose the Customers tab.
2. Click the customer's name and hit Edit. 
3. Tap Save. 

For more information, visit this article: Edit customer information.

See the sample screenshots below for the new invoices and old invoices: 

However, if you are referring that the parent customer's new email address will flow to your sub-customer, this isn't flowing. Each of the customers has a unique email address to make sure to send specific transitions or notifications. 

You can always open different customer's reports by going to the Report tab. In this way, you can track or manage your customer's transactions. For more details, may check out this link: Popular custom reports in QuickBooks Online.

Please visit this article to learn more about how to email a sales form or report to multiple email addresses in QBO for your future reference. 

Let me know if you are referring to something else by clicking the Reply button below. I'm always here to help. Have a great day ahead!