There are no built-in options to customize the 404 page. You can only change/translate the text from that page using the default translation files (.po and .mo), found in /wp-content/themes/cleanlab/language/.
You can use a simple edit application like Poedit.
If you need to change more than the text, then you’ll have to manually modify the 404.php file found in the root folder of the theme.
Let me know if there is anything else I can help you with.