This article covers how to set up error pages in cPanel.
Log in to your cPanel: https://YOURDOMAIN.com/cpanel
Click on “Error Pages” under the "Advanced" section:
Choose the domain you want to set up error pages for:
Choose “Edit Common Error Codes” or “Show all HTTP Error Status Codes”.
You can edit an error page by clicking on it:
You can insert HTML code in the edit box as well as any extra information using the options at the very top.