Did you try this link? Once you put in your preorder information it comes up with a screen to modify your payment.
https://preorder.verizonwireless.com/iconic/iconic/screens/IconicSearchInit.do
I wish I knew about it, I had to cancel an order from 6/14 on 6/26 and reorder because of a credit card change. I lost my place in line because of it.
The modify payment does NOT work. I've tried over 20 times over the past 3 days ""Due to high volumes, we are unable to update your payment details at this time. Please try again later."