To download a version of an app that has not been released, first navigate to the App Versions page shown below. From this page, select the platform you wish to download by clicking the 'Edit' hyperlink to the left of the appropriate platform version.
App Versions Page
In the example below, the 'Edit' hyperlink to the left of android 3.6.0 was selected and the browser was redirected to the App Versions Edit page for the Android platform. From this page you can download an unreleased app by clicking on the 'Download' hyperlink under the Available Apps section.
App Versions Edit Page