Add a bit of Text to existing 404 page

Hi. I’m building a florist website using GP’s “Prime” e-commerce starter site with WordPress, GeneratePress Premium & GenerateBlocks.

I would like to make add a bit of new/extra text to my current (default) 404 Page (see example at https://isanflowers2.com/404/)

I probably just want to add a subheading saying “404 Error” above the text “It looks like nothing was found at this location. Maybe try searching?”

I’ve found the 404.php file in my theme’s folder on my web server but it contains what looks like functions &/or hooks, and I don’t see how to add the above new/extra content.

Please could you tell me how I can do that, without creating a brand new custom 404 page?

Thanks in advance