You have to select which ticket you will make the exchange for. TCP Exchange displays all ticktes that are attached to the PNR. Tickets are marked in colours represented by Coupon status (OPEN, VOID, EXCH, RFND, etc.):
Example with VOID and OPEN status ticket
You can display the ticket by clicking on "i"
Click on EXCHANGE to start the exchange process. TCP Exchange will offer 4 exchange options.