shader models are not software based but are based on the physical hardware of a graphics card and as such they cannot be downloaded. to upgrade the shader model you must buy a new graphics card. modern gpu's are up to shader model 5 i believe with DX11
If you cant play a game, and are getting a message about shader model 3, it is because you need a graphics card that supports it. In that case you have to get a new graphics card
It is computer code used by game programmers when they make games, and you have to have a video card that supports it to play games that use it.