Update order meta with custom field value

functions.php

<?php
/**
 * Update the order meta with field value
 */
add_action( 'woocommerce_checkout_update_order_meta', 'some_custom_checkout_field_update_order_meta' );
 
function some_custom_checkout_field_update_order_meta( $order_id ) {
    if ( ! empty( $_POST['some_field_name'] ) ) {
        update_post_meta( $order_id, 'Some Field', sanitize_text_field( $_POST['some_field_name'] ) );
    }
}
?>