The 404 page is generated from layouts/404.html. That file references the variables 404_title and 404_info that are defined in e.g. themes/hugo-theme-bootstrap/i18n/en.toml. To edit those variables, create the i18n folder in the site root directory and copy e.g. themes/hugo-theme-bootstrap/i18n/en.toml to it and edit the values of the variables.