Just make sure you delete it from “Apps” and then remove it from your iDevice. Then don’t forget to hit “Sync”.
Then try disconnecting the device and then closing iTunes.
Open iTunes back up, double-click the .ipa file (the new one) to install the new version of the Alpha test, install it via iDevice, and then hit “Sync”.
That should be it!
Let me know if you have any questions.