How to selectively restore save game data from old iphone backup?
I found the answer with iMazing https://imazing.com .  


What I did was copy the entire iTunes backup folder from my USB drive to the iTunes backup folder in my machine.  
Then I ran the iMazing App and went to the iOS App (app has to be installed) in the connected phone.  
Then I right clicked the iOS App and select extract.  
At this point, I selected extract from current backup (which would be the historical backup containing the app data I want to restore) and saved the extract file to my desktop.  
Once done, I select "manage apps > restore" from the bottom of the screen and select the file I just extracted.  
Phone restarted after restore and it was all restored perfectly!


If you get stuck, they are very responsive on twitter, which is where I got help to do this.