How to change the label text in default setting pages?

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;
}