I don't think there is a way to delete a build just like Mark Philip said.
I am using Xamarin. I fixed my problem by doing this in Xamarin:
project -> options -> IOS Application -> build
Edit: You just can't. I imagine you will have one chance per year to clean up builds like they do with registered devices, or maybe they just plan to keep them there forever.
You cannot delete a Build from Itunes Connect. Your project info file contains two attributes for referencing the Build version of your app, i.e., Bundle version and Bundle versions string, short. Usually both attributes have the same value (1.0.3) but you can upload to the Itunes Connect multiple Builds for the same version of the app just changing the second value.
So, you create a new version of the app in Itunes Connect (version 1.0.3) and set in your NewProject-Info.plist:
Bundle version: 1.0.3.
Bundle versions string, short: 1.0.4.
Archive and upload.
Once Itunes Connect processes the new Build, it will appear in the Pre-Releases section and you will be able to select it for the 1.0.3 version and send it for review.
The new version of the iTunes Connect App for iOS devices does not seem to have the "Reject Build" capability as did previous versions of the app and the iTunes Connect Portal. You cannot reject or delete builds uploaded to iTunes Connect in the latest versions (Dec 2014).
The solution is to just upload a new build. However for iTunes Connect to accept the new build, you must first increment an internally visible build number before recreating the archive.
The number to increment is labeled "Build" or "Bundle Version" depending on where you look in the Xcode interface. A change in one location obviously is reflected in the others.
Here are the locations where you can change the Build/Bundle Version number:
General > "Build: 1.0.1"
-or-
Info > "Bundle Version: 1.0.1"
-or-
Info.plist > "Bundle Version: 1.0.1"
Once the number is changed, re-archive the build and re-submit it to iTunes Connect.