You can void and recreate the payment in your Bill.com account. Then contact their customer service to guide you on how to change the vendor's bank details and get back the payments that were taken out from your bank.
In the left navigation menu, click Payments Out.
On the same line item as the payment, under the Actions column, click Void.
Select to Remit or Reissue.
Enter a reason for the void.
Click Submit Request.
You can check this link for more information about voiding payments for your reference: How to void a payment.
Also, you can visit their support site, and enter a brief description of your concern in the search field. You'll be routed on how you can update the bank information of your vendor.
Keep in touch if there's anything else that we can do for you.