How the change the default payment method

Hi,

The default one will be the first one available to the user.
So in the System>Payment methods menu, make sure that the order of your payment methods is as you want it.
Note also that once a payment method is preselected or selected by the user, if a new payment is made available to him for the same user session / cart, even if it is placed as the first one, the system won't change the selected payment method for him.
So when you test, you need to delete the cart in the Customers>Carts menu of the backend, and add again a product in the cart before accessing the checkout, to make sure that the new default payment method is properly selected.