Changing a monthly payment plan amount

I have a client that is on a monthly payment plan. They pay $1666.66 each month. They made half of a November payment for $833.33. They want to roll the remaining $833.33 into their monthly payments. How do I change the monthly payments to reflect this?

If you click on the link for the subscription, you can edit the monthly payment and the future payments will be billed at the higher rate.