1.
How to update your 404
WordPress page
By @ctrappe

2.
Space to update! (Depends on your theme)

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Compare to page!

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Things to keep in mind:
If you update the theme these changes get overwritten. (This is not the
child theme process.) I don’t see that as a huge issue in this case,
because when the theme pushes out an update, just update your 404
page again with something new and interesting. It’s a good reminder to
keep even the 404 page fresh. And no, do not skip theme updates!
@ctrappe
AuthenticStorytelling.net