Welcome to the Community! Once the first payment is made, you can not modify the payments. However, you can cancel the second payment request and create a new payment request. Here's how: https://help.homeaway.com/articles/How-do-I-create-a-payment-request
HomeAway Community Manager
ACK! I am loathe to contradict Erin, but there are so many complications that have happened when I made the mistake of cancelling the second payment and re-creating a new one for a different amount. I don't think the negative experience is only mine..... There was a discussion some time ago in which this was discussed ... I'll see if I can find the link. Basically, it causes confusion for guests, and also creates issues with the automatic refund of the security deposit.
WAY BETTER OPTION! In my experience, it's better to just refund the discounted amount back to the FIRST payment. So, for example, I require 50% up front and 50% 30 days prior. Let's say a guest makes a 4000.00 booking and then makes a first payment of 2000.00 but for whatever reason I give them a 10% discount. I would go back and refund $400 to the first payment and they will make their 2nd payment (which in my case includes the security deposit) as usual. Then they get the security deposit automatically, as usual.