There are a few ways to do it. The easiest way appears to be this – https://wordpress.org/plugins/genesis-404-page/ This plugin was created by Bill Erickson (a well known Genesis Developer) and he usually creates good stuff if you want to give that a try.
The 404 page is located inside of the Genesis Framework (not the child theme)…..so you can copy the 404 file and add it to your child theme folder/directory and then you can edit it that way.
If you want some tutorials you can Google something like – “custom genesis 404 page” and you’ll see a lot of tutorials that cover how to do it.
Let me know if this helps.
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