How do you switch recurring invoice from card payment to ACH payment?

[limbmma8]

I have a customer that would like to switch from paying by card every month to bank draft instead. The customer is having some difficulty with their bank in getting an updated card as the one on file has expired. I can't figure out how to change the recurring invoice from card to ach payment. I have the authorization form with the info needed. How can this be done? Thanks.