If you want to reinstall the first edition Sibley eGuide app on your device, you will discover that it is no longer listed in the app store. You can still find it in your purchase history.
With the release of the new second edition of the app, the first edition is no longer being supported or sold, and has been removed from the catalog in the app store. It is still available for download to anyone who already owned it. Simply log in to your app store account and view your “purchased” apps. The Sibley Guide to Birds v1 should show up there and be available for download.
Even though the original app is no longer being supported it should continue to function for some time, but as the the operating systems of phones are updated the app will simply stop working at some future date. If you continue using it, be sure to regularly back up any list data that you keep in the app.