How do I change the email address from which the invoices are sent out to the clients?

You can change the from email address from which the invoice are sent out by configuring your new email address in IMAP of "Preferences" setting. Make sure that the configured email is in active state and authenticated.

Make visible the secondary email address

1. Log in to your account, configure your email address in "Your account name at the top right corner -> Preferences -> Email".
2. This leads to add your email account into IMAP sync, which syncs incoming emails and outgoing emails.
3. Now navigate to Invoices App, to view the added email address or secondary email address. 

Steps to change the email address

1. Select an invoice that you would like to send it to customer. 
2. Click on "Send" blue button on the invoice overview page. 
3. You will view "Compose" pop up with email related fields. 
4. Now, click on "From" drop down you will view business email address as well as configured email address.