A function that modifies the default WooCommerce orderby dropdown. You can put this snippet in your functions.php file.

woocommerce-modify-orderby.php

<?php
// Modify the default WooCommerce orderby dropdown
//
// Options: menu_order, popularity, rating, date, price, price-desc
// In this example I'm changing the default "Sort by newness" to "Sort by date: newest to oldest"
function patricks_woocommerce_catalog_orderby( $orderby ) {
	$orderby["date"] = __('Sort by date: newest to oldest', 'woocommerce');
	return $orderby;
}
add_filter( "woocommerce_catalog_orderby", "patricks_woocommerce_catalog_orderby", 20 );