To change the coupon code for any of your affiliates, just head over to the Affiliates tab.
Click the small info icon next to any of your affiliates:
That should open a dialog that displays the current coupon code for that affiliate, along with a link that says "Change coupon code":
When you click the link, another dialog will open to let you edit the coupon code and save it. Just keep in mind that the old coupon code will stop working immediately!