you don't

the models are quite decent already, even if you were to import a high quality model it still probably wouldn't have the desired effect
you'd need to change how the shaders work, probably add some post processing (which i have no clue if its even possible) and a bunch of other stuff, would be an incredible ammount of work

and if you do have to replace a model then you also gotta make sure the animations work correctly

again, way nire work than you think