How to Set Custom error pages In Cpanel.

To create a custom error page for your account, please login to cPanel and click Error Pages, under Advanced. Select the domain or subdomain you want and click the page you want to edit. Insert your own custom page code (in HTML or SHTML). The changes will be applied after you click Save.

Common problems:

If you are still seeing the old error pages, try refreshing your browser (ctrl + F5). If you still see the old pages, please add this code to your .htaccess file in the public_html directory.

ErrorDocument 403 /403.shtml
ErrorDocument 404 /404.shtml
ErrorDocument 500 /500.shtml                                            

You can even use existing pages as the resulting page. Say you don't want any visitors seeing 404 errors on your site. You can make your home page the result, like so:

ErrorDocument 404 /index.html
If Internet Explorer is not displaying the custom error page, it is likely because the error page must be larger than 1 kilobyte.

There are many error pages which may be defined.

Client Request Errors
400 - Bad Request
401 - Authorization Required
403 - Forbidden
404 - Not Found
405 - Method Not Allowed
406 - Not Acceptable (encoding)
407 - Proxy Authentication Required 
408 - Request Timed Out
409 - Conflicting Request
410 - Gone
411 - Content Length Required
412 - Precondition Failed
413 - Request Entity Too Long
414 - Request URI Too Long
415 - Unsupported Media Type

Server Errors
500 - Internal Server Error
501 - Not Implemented
502 - Bad Gateway 
503 - Service Unavailable
504 - Gateway Timeout
505 - HTTP Version Not Supported

Q. How can I revert back to the default error pages?

Simply delete the error page from the public_html folder:
404.shtml
403.shtml
500.shtml