You can modify the labels using gettext
filter hook ex:
add_filter( 'gettext', 'theme_change_label_names');
function theme_change_label_names($translated_text){
if (is_admin()){
switch ( $translated_text ) {
case 'Site Title' :
$translated_text = __( 'New Site Title label', 'theme_text_domain' );
break;
case 'Tagline' :
$translated_text = __( 'new Tagline label', 'theme_text_domain' );
break;
}
}
return $translated_text;
}