if ($order->getInvoiceCollection()->count()) {
    $_invoices = $order->getInvoiceCollection();

    if ($_invoices) {
        foreach ($_invoices as $invoice) {
            $invoice->delete();
        }
    }
}

$items = $order->getAllItems();
foreach ($items as $k => $item) {
    echo $item->getId() . "n";
    $item->setQtyInvoiced(0);
    $item->save();
}