Swapping a customer on the Reservation Detail Panel (RDP) couldn't be easier. 

Click on the swap icon at the top of the RDP next to the customer name:

This will bring up a customer search. Enter at least 3 letters and press search, the results will be returned below. You can double-check you have the right customer by clicking on the right-hand icon (see blue annotation in the screenshot below).

Then simply click on the customer you'd like to swap. You'll then be asked to confirm the swap and the reservation will be updated with the swapped customer's details.