I have been using testflight to update my (iphone) app. I want to go back to an earlier build. Is there a way I can download an old build from testflight and access the contents (I need access to a plist file).
This answer is deprecated with the shutdown of Testflight.
You can access a plist file, but you can not access source code. I've updated the question title to reflect what you're trying to do. :)
From your list of apps, click on the one you'd like to download, then click on the build. On the left, click "Permissions", scroll down, and click the link under "Share This Build". On that page, click "download the IPA" under manual installation.
An IPA file is actually a ZIP archive, so change the file extension and unzip it. Inside the Payload directory you'll find a App file. Right click on it and select "Show Package Contents". Here you have all the resources being used by the app, including your plist.
Note that you can not access the source code in this manner because it is compiled into ARM machine code.