How to change login labels

All of these strings/labels are passed through translation functions, so you can use gettext filter to modify them.

function change_labels( $translated_text, $text, $domain ) {
    if ( 'Username' === $text ) {
        $translated_text="Username new label";
    }
    return $translated_text;
}

function register_change_label_filter() {
    add_filter( 'gettext', 'change_labels', 20, 3 );
}
add_action( 'login_head', 'register_change_labels_filter' );
// this way our filter will work only on wp-login and not everywhere on site...