When an ambassador needs to change the payment details they gave you, clear the old ones so they can enter new ones.
Go to the Ambassadors tab, find the ambassador, click the Edit icon, then click Reset next to their payment information.
The next time they log in and open their payouts, they'll be prompted to enter their payment details again.
One thing to note: resetting doesn't send them an email, so let them know to log in and add the new details.