How can i get OrderTotal and edit OrderTotal?

Hi firends,
I'm using nop v2.3 and try to write a new payment plugin. I must get OrderTotal and then edit it.
I try all way, recalculate, cart items etc..

In Controller "public ActionResult PaymentInfo() " I want to get OrderTotal. And then In controller "public override ProcessPaymentRequest GetPaymentInfo(FormCollection form)" i want to edit order total.

If you dont understand what i said. can u look sadecedizustu.com? In checkout page step 4 select "kredi kart", step 5 select installment and write owners credi cart information. in step 5 there are installment's ratio. Here i want to calculate order total and write OrderTotal x ratio And then step 6 i want this OrderTotal calculate: OrderTotal+=OrderTotal*Ratio

Shortly how can i get OrderTotal? How can i edit OrderTotal?

I try these in public override ProcessPaymentRequest GetPaymentInfo(FormCollection form)


            var paymentInfo = new ProcessPaymentRequest();
            paymentInfo.CreditCardName = form["CardholderName"];
            paymentInfo.CreditCardNumber = form["CardNumber"];
            paymentInfo.CreditCardExpireMonth = int.Parse(form["ExpireMonth"]);
            paymentInfo.CreditCardExpireYear = int.Parse(form["ExpireYear"]);
            paymentInfo.CreditCardCvv2 = form["CardCode"];
            paymentInfo.OrderTotal=ConvertToDecimal(10); // for test
            return paymentInfo;

OrdrTotal not change? Why not change? I didn't find