How can I change a customer’s billing information from credit card to ACH?

Adding Bank information to be stored on a customer's account after signup requires that the hosted pages are enabled and that the Customer View setting is set to Account Login, by default this may be showing as Guest View instead. Once that's been done, you'll want to visit the customer's account page and on the right-hand side of the page locate the Hosted Pages section and use the Hosted Account Management URL. This will provide you with a link that you can provide to the customer or use yourself to access their hosted account page. On the page, simply click Change Payment Method and then Bank account to edit and change the payment info to use ACH bank payments.