I only ordered 1 item on the auction. my bid was successful. Can I change my quantity to 2 items & double my payment.! How to I edit mytotal

[dezzyboy97]

I bought  1 item on Ebay, my bid was successful. I would now like 2 of those items. How do I change the quantity and payment total.  I've Emailed the seller and he said just double your payment total.  Again, How do I change the total payment in the payment section ?