In search try to find Report Selection - Sales

Make sure you select Invoice under Usage tab

Click on Email Body Layout. You see all available templates. Find the report ID and the Layout name.

After that go back to search and open Custom Report Layout page

Filter layouts with the ID or description

We want to modify Fact. Invoice e-mail template. Lets select it and click Home → Layout → Export Layout. Open the downloaded file

Make changes in your template

Go back to the Custom Report Layouts page. Make sure you selected the same layout. Usually blue color identifies which layout is selected.

Click Home → Layout → Import Layout

Drag and drop or select the file

Try to run report to see the changes.