How to update custom field in WooCommerce

Okay found it. It was a silly mistake I overlooked. I had 1 attribute (esc_attr($myField)) double inside the update_post_meta() method.

So this function works perfectly:

function woocommerce_save_fields($post_id)
    {
        $myField = $_POST['field_name'];
        if (!empty($myField))
          update_post_meta($post_id, 'field_name', esc_attr($myField));
        else update_post_meta( $post_id, 'field_name', '' );

    }