how can you make changes to a pc game?

Answer 1

There are many internet sites where people produce programs called "mods". These can do anything from simply add a few choice items to the game, or alter its look entirely.
To find and download one, do an internet search on the name of the game you want to modify, along witht he word "mod"

Answer 2

Aside from mods, it is also possible that the game supports .xml editing, such as Civilization IV. All commercial games, however, are not open code and changes must be made indirectly, as mods do.