The default index page is the page that will be placed by default to your newly-opened hosting accounts.
To edit the default index page template, you need to go to the "Account & Clients" > "My Account" section of your Account Panel, then click on "Custom Templates", and on "Customize Index Page". There, you can make changes to the HTML source of the default index page. Do not forget to click on "Update Template" to save the changes you made.