Hey guys,
I recently had to update my payment information as I just moved apartments, and it turns out this is kind of a pain. There's no way to just change information about the current default payment option, and you have to completely make a new one. If there is a method to do this, it's not easily discernible from the My Payment Methods module.
It also seems that if you have a default method already existing, you have to remove it before you can actually save the new method, and the way you save the new method is by clicking on 'use this address'. Since the button is contextually linked to the address portion of the form, it doesn't really make sense that that's the button you use to save the card information as well.
You should get the latest minion to poke at this! :)
Cheers