Where to edit invoice email template?

When you open an order by clicking the invoice number there is a button called "email" in the toolbar. It displays in a popup an email text. In which email template can I edit this text and layout?

Hi,

The text of the email in that popup can be modified with translation overrides:
www.hikashop.com/download/languages.html#modify

The layout of the email, or if you want to add new texts, you can do so by editing the email "order notification" in the menu Display>Emails.