but I saw someone did it and the game was runing about 20 fps
Link? We can't see things you saw by the simple fact that you saw something. What were the settings used? What was the exact equipment used across the board?
And I'm not sure 20 FPS is something you want to strive for.
The fundamental problem here is that you have what is likely a 15-year-old office PC in a small form factor. Pretty much any worthwhile upgrade will involve replacing the entire PC, because you don't even have the basic framework upon which to make changes. You'll also need to provide exact
details of what your hardware is. All you've told is that you have an office PC with a CPU that was made across a four-year period from 2005 to 2009.
better off simply buying a used PS4 if you don't already have one. It'll be a far
better experience than anything you'll be able to do with this office PC.