The swap option is only available for the Silver Annual subscribers.
As a Silver Annual subscriber, you have the choice to select either a CPTS or CBBH voucher to redeem for an exam. You are allowed to freely exchange vouchers until you make a final decision on which one to utilize.
To do this, you need to click the voucher icon under your avatar, choose your current exam voucher and select the one to exchange for. You can then finalize using the Exchange Vouchers button and Proceed.
You can swap freely between vouchers until you use one of them.