Hi, same issue, for Mac QB 2019.

I made the edit to the account number, but got a statement from the bank that the account number is wrong. (But I'm quite sure it's correct.)  When I click on the "Online Settings" button in the Edit Account window, it shows the wrong (old) credit card number, but no way to edit it.

Do I have to delete and then merge? Can you supply instructions for doing this on the Mac. The one is this thread seems to be for PC only. 

Thanks!