• We have updated our Community Code of Conduct. Please read through the new rules for the forum that are an integral part of Paradox Interactive’s User Agreement.
The save file will be half updated and half old, which is to say irredeemably corrupted. There is no possible way to fix that, I am sorry.

In future I'd suggest you manually make a copy of your Ironman saves, either just before or just after each play session.
 
How do you copy a file in Windows, are you asking?

Right click on it in File Explorer, copy. Go to a different folder, right click in there, paste.