Log in as root to your WHM.
Now in the search bar, search for Web Template Editor.
Here, you will find 4 tabs. It enables you to upload your HTML page or to add the HTML source code.
After updating the press Save Button.
Add Default Index HTML Page on new account creation
Login to root user of the server or use the Terminal option from WHM.
Copy HTML and Images to `/root/cpanel3-skel/public_html` for cPanel accounts whose owner will be root.
If you have any reseller with the username user and want their accounts to have custom HTML page then paste the index.html file in `/home/user/cpanel3-skel/public_html`
Any new accounts created on the server will get your custom HTML page in its public_html directory automatically.