Woo_checkout_reorder_fields.php

<?php
/*
* Reorder WooCommerce Checkout Fields
*/

function ci_woo_reorder_fields($fields) {
$fields['billing']['billing_first_name']['priority'] = 10;
$fields['billing']['billing_last_name']['priority'] = 20;
$fields['billing']['billing_email']['priority'] = 30;
$fields['billing']['billing_phone']['priority'] = 40;
$fields['billing']['billing_country']['priority'] = 50;
$fields['billing']['billing_company']['priority'] = 60;
$fields['billing']['billing_address_1']['priority'] = 70;
$fields['billing']['billing_address_2']['priority'] = 80;
$fields['billing']['billing_city']['priority'] = 90;
$fields['billing']['billing_postcode']['priority'] = 100;

return $fields;
}
add_filter('woocommerce_checkout_fields','ci_woo_reorder_fields');