If you're unable to update the bank information, we can do a few troubleshooting steps. Cache might have contributed on this unexpected result that we have right now. Though it helps your browser's background processes run smoothly, this can also cause unexpected issues when piled up. Here's how to get rid of any cache-related concerns:
Open a private browser and log in to your account. It uses but doesn't store cache.
If you can update the vendor's bank information successfully, we can go back to your regular browser and clear the cache.
If none of the steps above work, I'd recommend contacting our Customer Care Team. They can review the details and help you update the vendor information. You can reach them through this link: QuickBooks Online Support.
Let us know how this goes so we can further assist you.