These fields have the keys 'blogname' and 'blogdescription' respectively in the options table.
I’m not sure exactly why you need to do this as it seems like a priority to secure your site and be able to update it through the dashboard yourself, but you can update them easily from functions.php with update_option, e.g.:
update_option( 'blogname', "site title goes here" );
update_option( 'blogdescription', "site tagline does here" );