Cancel a completed order

[smaarthomes]

Hello, 

I am running magento 1.9.2.4 , I am using the first data payment gateway for ICICI. When a guest user places an order on the website and makes the payment, the order status automatically shows completed and the invoice gets generated. Now if the user wishes to cancel their order, I am unable to update the same via the admin panel. I can process the refund from the gateway interface but the order status in my admin panel shows completed. 

How can I cancel an order once the invoice is generated ?

[Pronto]

Once you have created an invoice, only way to cancel it is to issue a credit memo: invoice is basically a legally binding document and only way to void it is to use credit memo.