WHM >> Main >> Account Functions >> Web Template Editor
Here you can change the default page, suspended page etc
Look for defaultwebpage
References:
https://stackoverflow.com/questions/5271749/change-default-page-in-cpanel
https://www.motherhost.in/members/knowledgebase/22/how-to-change-default-page-in-cpanel.html
https://forums.cpanel.net/resources/why-do-i-see-the-default-web-site-page.489/