We submitted our binary, but discovered that there was an error. I uploaded a new binary from XCode6 and it shows up in the Prerelease Tab of iTunesConnect. It has a higher build number (#706) as opposed to the "waiting for review" version (#704).
I can't reject #704 and replace it with #706 in Waiting for Review. There is no reject binary button anywhere in the iTunesConnect UI, nor on the mobile app. According to the Apple Documentation it should be here, in this screenshot: