In that folder will be all of the current apps plus a folder called Previous Mobile Applications.

Go to the Music>iTunes>iTunes Media>Mobile Applications folder on your drive.

Go to iTunes and Delete the new version of the App from your library and also from your drive.

This is unfortunately not going to help you with this app, as it does not sound like you had downloaded this app prior to this question, but it will help you in the event that you accidentally upgrade an app that will then no longer work with your version of iOS.

If all of this fails, possibly other options like Todoist have the older version of their app that you can access. They may be able to let you directly download the version of the app that you need from them. Click that and then put in a query to the developer.

There is a button there that says App Support.

If the version is not available, go to the iTunes App Store, find the app and click Ratings and Reviews.

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.

Press that and it will show you all of your apps.

When you get there, it should show your Apple account and it will say.

Press Updates and then press Purchased.

It sounds like the Apple Support article explains what you need to do to get the app in the version you want.

Free for apple instal Old Snook