WC_Product_Data_Store_CPT::update_product_stock PHP Method

    public function update_product_stock($product_id_with_stock, $stock_quantity = null, $operation = 'set')
    {
        global $wpdb;
        add_post_meta($product_id_with_stock, '_stock', 0, true);
        // Update stock in DB directly
        switch ($operation) {
            case 'increase':
                $wpdb->query($wpdb->prepare("UPDATE {$wpdb->postmeta} SET meta_value = meta_value + %f WHERE post_id = %d AND meta_key='_stock'", $stock_quantity, $product_id_with_stock));
                break;
            case 'decrease':
                $wpdb->query($wpdb->prepare("UPDATE {$wpdb->postmeta} SET meta_value = meta_value - %f WHERE post_id = %d AND meta_key='_stock'", $stock_quantity, $product_id_with_stock));
                break;
            default:
                $wpdb->query($wpdb->prepare("UPDATE {$wpdb->postmeta} SET meta_value = %f WHERE post_id = %d AND meta_key='_stock'", $stock_quantity, $product_id_with_stock));
                break;
        }
        wp_cache_delete($product_id_with_stock, 'post_meta');
    }