Update the order meta with the field value

functions.php

<?php
//* Do NOT include the opening php tag shown above. Copy the code shown below.

//* Update the order meta with field value
 add_action('woocommerce_checkout_update_order_meta', 'wps_select_checkout_field_update_order_meta');
 function wps_select_checkout_field_update_order_meta( $order_id ) {

   if ($_POST['daypart']) update_post_meta( $order_id, 'daypart', esc_attr($_POST['daypart']));

 }