如何在支付发票和处理订单状态时取消订单？

I am facing problem with magento store. I am using payperrental extension for rent. Actually I am unable to cancel order after when invoice status is paid.

So how I change the order status and also cancel order?

you need to create credit memo for that order by going to order view page and on top you will find one button called credit memo

After generating credit memo your product inventory will go back to catalog.

Brief Regarding Credit Memo

After an order has been paid and invoiced, all of the order, or a portion of it, can be refunded by issuing a credit memo. A credit memo differs from a refund because the amount of the credit is restored to the customer's account where it can be applied to future purchases, rather than returned to the customer as cash. In some cases, a refund can be given at the same time that the credit memo is issued, just as a payment can be made at the time of invoicing.