<?php
// Adding a hook callback.
add_filter( 'woocommerce_default_catalog_orderby_options', 'custom_woocommerce_catalog_orderby' );
add_filter( 'woocommerce_catalog_orderby', 'custom_woocommerce_catalog_orderby' );
function custom_woocommerce_catalog_orderby( $sortby ) {
	$sortby['title_asc'] = __('Alphabetical - (A - Z)', 'your-theme');
	$sortby['title_desc'] = __('Alphabetical - (Z - A)', 'your-theme');
	return $sortby;
}