To transfer a payment between reservations:
In the reservation that has the originating payment, click on the Transactions tab along the left side.
In the Transactions detail screen, find the payment that you're looking to transfer funds from. On the right side of that line, there should be an icon with two arrows.
After clicking on the Transfer button, you'll see the following screen:
If the reservation receiving the funds is under the same contact name, future reservations will show up under "Related Reservations". Click on the circle button, confirm the amount that will be transferred in the Amount box, and click the Transfer Payment Button.
If the reservation receiving the funds is under a different contact, click Override. Review the amount being transferred, and then enter in the destination reservation's reservation number in the box below. If everything looks good, click Transfer Payment.