To download older version of app again because you're not able to run the most recent one:
Go to the App Store on your device.
Press Updates and then press Purchased.
When you get there, it should show your Apple account and it will say My Purchases.
Press that and it will show you all of your apps
Find app and download it.
If a version of the app that is compatible with your version of iOS is available on the App Store, it will let you download it to your phone.
If you just want to use a previous version while you still are able to download the most recent one, please see http://www.hightechdad.com/2015/03/26/how-to-restore-a-previous-version-of-an-ios-app/