Good day, Jevans. 

You'll need to change the payment method first from Online Bill Pay to paper checks and toggle it back within QuickBooks Online. Once done, the system will ask you to re-enter their bank account information or sending them a new request.

For the detailed instructions, I'm adding the article I find helpful on this:

Change a vendor payment method.

Please don't hesitate to keep in touch with me here should you have any additional questions or concerns about updating the vendor bank information. I want to make sure you're taken care of.