Games are only getting more demanding and if you game on your desktop it might be time to replace the graphics card. This will work on most windows desktops but not all, and definitely not any Apple computers or laptops.
Step 1: Choosing Your Graphics Card
There are a Hundreds of different graphics cards mainly AMD and Nvidia's creations, however other companies also take these graphics cards and add little features. What you get is your choice - you can go high performance and pay upwards of 1000 dollars, there is even a 3000 dollar graphics card. Or you can buy a lower end graphics card and pay as low as 50 bucks. The middle ground is usually the place to go where you get the best performance per dollar a good example of this is an gtx 950 by Nvidia.
Step 2: Opening the Computer
This is pretty easy, some computers you have to screw open but most have the ability to open from the side.
Step 3: Removing and Putting in the Graphics Card
The graphics card is a rectangular usually thick piece of hardware usually card shaped, the picture above should give you a basic idea of what it looks like, now remove it gently - no force should be used, and add the new one also gently, you have just replaced your graphics card.
Step 4: Wrapping Up
You should be good to go if you bought an upgrade you will most likely see a noticeable improvement. Have fun gaming.