Disable or custom page 404

Hi, I’m trying to customize the page 404 (Like fully customize it, and it’s not in the options offered in the Highend Options), because the title in the tab is like “Page not found”, but my website is in french.

I didn’t find a way to make advanced modifications to this page, so I wanted to create a new one altogether, but I need to disable Highend’s default one first.

How could I do any of both ?

Thanks in advance, have a great day !