I\'ve just uploaded my application to the appstore. The application requires a username / password to log-in, but I was never prompted for any credentials during the upload proc
App demo account details can be updated under "App Review Information" in iTunes connect as shown below image. Even the details can be provided under "Notes" in iTunes app information
When you submit the app, there is a field near the bottom of one of the screens for "review notes" which is where login information should be placed. You may be able to in to iTunes connect and edit this field. I have no apps waiting for review at the moment, so I am unsure if this field is editable, but it should be.
The bottom-most field on the page where you edit your app's metadata (title, description, keywords, etc.) is titled "Review Notes (optional)". It is exactly for your purpose. I don't know if you can still edit it after you have submitted the app for review.