How do I change the payment options on my Amazon Account

I have three different credit/debits cards on record for my Amazon account. I use the Barclaycard credit card as the default card. Recently I got a new Barclaycard which I wish to make the default card, to replace the existing card. So I added it to my Amazon account.

I then made it the card to be used for one click processing.This did not work, the default card remains the existing account which is still valid. I have a couple of o/s orders on it and I want them to finish processing first so I suppose I could leave it till then.

However sometimes I wish to use one of the other "valid" cards as the default card and I do not see how to do this. Where is the setting to use for making a particular card the card to be used for payment of a particular purchase?