How do I change WP search results order?

You can use the action hook posts_orderby in your child themes functions.php:

function changeSearchSort( $orderby, $query ){
    global $wpdb;

    if(!is_admin() && is_search()) {
        $orderby =  $wpdb->prefix."posts.post_date ASC";
    }
    return  $orderby;
}
add_filter('posts_orderby','changeSearchSort',10,2);