Override / set / replace / clear WooCommerce checkout page field values

woo-checkout-fields-edit.php

<?php
/**
 * Pre-populate Woocommerce checkout fields
 * Note that this filter populates shipping_ and billing_ fields with a different meta field eg 'first_name'
 */
add_filter('woocommerce_checkout_get_value', function($input, $key ) {
    if($key == 'billing_first_name') {
        return 'NEW FIRST NAME';
    }
    if($key == 'billing_last_name') {
        return 'NEW LAST NAME';
    }
    if($key == 'billing_email') {
        return '[email protected]';
    }
    if($key == 'billing_phone') {
        return '0123456789';
    }
});
?>