Change apt field label

change_apt_label.php

<?php
// Do NOT include the opening php tag.
// Place in your theme's functions.php file

/**
 * CheckoutWC 2.18.0+
 *
 * Change shipping and billing address 2 label
 */
add_filter( 'woocommerce_default_address_fields', 'custom_override_address2_label', 200000, 1 );

function custom_override_address2_label( $address_fields ) {
	$address_fields['address_2']['label'] = $address_fields['address_2']['placeholder'] = 'Apartment Number';

	return $address_fields;
}