How do you lower the graphic settings on a computer game?

For example, Napoleon Total War. I Want to make the game be able to run on my computer but my graphics aren't good enough.How do you lower the graphic settings on a computer game?
Usually games have two ways to alter graphics performance. There is either an out-of-game setup program that you select graphics options from, or you have to go to the in-game options menu and alter the settings there. When you run the game, does it run cruddy or does it not run at all. If it doesn't run at all, what does the computer tell you when you try to open it?