Changing the default Error Page in Joomla

You can create a customized error page for a template in Joomla by copying your default page to your template files. This will override the default template.
1. Open Windows Explorer and browse to the folder "Templates" in your joomla site’s root. Go to the folder "System" and select the file "error.php" . Now copy this file.
2.Browse to your theme folder inside the "Templates" folder, and at the root of the site files, paste the "error.php" file that you just copied to override the same named file in the system folder.
3.Open the "error.php" file from the template folder using Notepad.
4. Edit your file as per your requirements. You can change the page’s style the style as well. Save the changes. And you’re done.