Hi @laniolsen
Welcome to Shopify and our community forums! To edit the 404 page, you would need to do so through the theme code. Under Templates you will find the 404.json template. I am not a trained developer myself, but this resource on our developer docs might help you in adding a text block or image: JSON templates.
I strongly suggest making a duplicate of your template to work on or to import a fresh version of the dawn theme to play around in until you get the results you are looking for. You shouldn’t make theme code edits on a live theme.