Change auto payment from credit card to bank account

I spent way too long  on Xfinity and Xfinity Mobile looking for how to simply change my monthly auto payment method from a credit card to a bank account.  My Xfinity monthly auto payment is already set up with my bank account.  I want my Xfinity Mobile bill sent there as well.  But how to do that?????  Seems simple but alas, after an hour, it is not.  Help please!  Rod