Magento Change Order Status Programmatically From http://phptalks.wordpress.com/2011/09/09/magento-how-to-change-order-status-programmatically/

Magento Change Order Status Programmatically

$orderId = YOUR_ORDER_ID;
$order = Mage::getModel(‘sales/order’)
->load($orderId);
If you have order increment id, you can load order in the following way:-

$orderIncrementId = YOUR_ORDER_INCREMENT_ID;
$order = Mage::getModel(‘sales/order’)
->loadByIncrementId($orderIncrementId);
Now, here is the code to change order status:-

/**
* change order status to ‘Completed’
*/
$order->setState(Mage_Sales_Model_Order::STATE_COMPLETE, true)->save();
Similarly, you can change the order status to pending, processing, canceled, closed, holded, etc.

/**
* change order status to ‘Pending’
*/
$order->setState(Mage_Sales_Model_Order::STATE_NEW, true)->save();

/**
* change order status to ‘Pending Paypal’
*/
$order->setState(Mage_Sales_Model_Order::STATE_PENDING_PAYMENT, true)->save();

/**
* change order status to ‘Processing’
*/
$order->setState(Mage_Sales_Model_Order::STATE_PROCESSING, true)->save();

/**
* change order status to ‘Completed’
*/
$order->setState(Mage_Sales_Model_Order::STATE_COMPLETE, true)->save();

/**
* change order status to ‘Closed’
*/
$order->setState(Mage_Sales_Model_Order::STATE_CLOSED, true)->save();

/**
* change order status to ‘Canceled’
*/
$order->setState(Mage_Sales_Model_Order::STATE_CANCELED, true)->save();

/**
* change order status to ‘Holded’
*/
$order->setState(Mage_Sales_Model_Order::STATE_HOLDED, true)->save();
You can also cancel an order in the following way:-

if($order->canCancel()) {
$order->cancel()->save();
}
Hold an order:-

if($order->canHold()) {
$order->hold()->save();
}
Unhold an order:-

if($order->canUnhold()) {
$order->unhold()->save();
}