Hi Said
You’d need to use a child theme. This will take some custom development so if you’re not up for that, it’s better to look for a custom 404 page plugin, there are a few out there.
Custom route would be to use a child theme. Make a copy of 404.php from the parent theme and insert it into the child theme and then edit as required.
The plugin route is easier: https://wordpress.org/plugins/search/custom+404/