How do I notify and/or force a beta tester to update their installed version if a newer version has been uploaded / distributed?
Using either Crashlytics (aka Fabric.io) or HockeyApp, this should be possible. At the moment, I only see a way to do it via email notifications, but I have seen others do it via an in-app notification.
On HockeyApp, when integrating the SDK, the app will automatically notify the user with an alert if an update is available and the user can then update from within the app. In addition you can set a new version as being "mandatory" in the web interface, then the user will then be forced to update from within the app and can not skip the update.