For each error in Joomla CMS, a "Joomla error detail page" or "404 error page" is displayed.
When the "Show Errors" setting is turned off in "Joomla's Global Settings" → "Show Errors" and some error occurs or the user enters the wrong URL in the browser, the user will see a "404 error page".
More about: How to "turn on" or "turn off" the Error report in Joomla.
1. With the help of FTP or cPanel or Joomla File mnager, go to {your-website}/templates/{your-template}/ and open file: error.php.
1.a If you don't have error.php file, Please go to {your-website}/templates/system/ or {your-website}/templates/cassiopeia/ and copy error.php file from there to your template folder.
2. In error.php file, at the file begin after row:
defined('_JEXEC') or die;
add code:
if (($this->error->getCode()) == '404') { header('Location: your-wedsite-URL'); exit; }
where: {your-wedsite-URL}, any correct URL on your website which you need for show users when 404 error happened.
3. If you "website developer" and know PHP, you may change {your-website}/templates/{your-template}/error.php file exactly as you need for any errors types or set this page any design.
4. For Website developers: If your website - will have real error, and you will need to debug it. Please go to {your-website}/templates/{your-template}/error.php and remove code from for 404 error page redirect which you added at steps before.
More about Joomla Errors show settings: