In order to cancel a payment method we will go to:
Customers -> Customers
We will see a window with the list of all the clients. We will click on the button
of the customer that we want to cancel the charges.
Inside the customer's editing window we will go to "Billing and Collection" in the menu on the left and in "Sales and Payment".
We can know if the ticket is already charged or not because we have a box with the name Paid.
That tells us, so if it is not marked, it is still to be charged.
In order to change a user charge, we will click on the button 
When we open the next window (shown below) we click on the button of the magnifying glass, (as shown in the image).
When we have clicked it will open a list with all the customers. We will click on the button
of the customer to whom we want to change the payment.
