The slogan is displayed in the box floating over the cover image on the homepage, and is configured via the $siteSlogan property on the Laddr PHP class. To change it, you can override php-config/Laddr.config.php.
You can open the inherited default version Laddr.config.php from v1.laddr.io under the _parent tree and make your changes directly there. When save your changes to the inherited file, a local copy will be created automatically at the same path in your local php-config tree.
You can also customize your site’s title and abbreviation from this config file.