The build number (not the version number) determines the order of versions for an app:
The build number is a unique value and we try and refrain from altering it. If a build number needs to be changed then the quickest approach is to remove the version in question, then re-uploaded it with the desired value. If deletion isn't an option then it is possible to alter this directly, but the turnaround time will be slightly longer.
Raise a request with the Marketplace support team and provide the details of the app and the version in question.