Q: how to install old version of an app ???

This isn't technically possible, but I did find a Cult of Mac article on it. The process isn't simple and probably isn't worth it, so you should consider if you really need to do so.

ArsI56,

This Apple Club article may be helpful.

You can take a look at this support document, Install the latest compatible version of an app on an earlier version of iOS or OS X - Apple Support, but if a newer version of an app exists, and works with the current iOS on your iPhone, then you would not be able to download the older version.

Okay, at that point, there really isn't anything I am aware of you can do. In Windows, if you update the app via the computer and iTunes, it will put the older version of the app file in the Recycle Bin, but that is not done if you update the app via the iOS device itself.