Hello, Peter Vaughn. 

We can locate the Edit button under Customers field. From there you can manually modify the customer email address. 

1. Click the Customers on the left panel. 
2. Select Customer and choose the customer name you want to change the email address. 
3. On the right side upper corner click the Edit. 
4. Enter the new email address of your customer. 
5. Press Save once done. 

Please browse through these articles below. These links contain modifying customer email addresses and on sales forms:  

• Change the email address your customers see in QuickBooks Online 
• Change your customer-facing email address 
• Manage your customer list

If you have other questions, please hesitate to go back to this thread. Have a great day!