First, you have to go to settings, and switch to the content tab, which only has about 3 options. At the top will be an option that says "enable original models" which is always on by default. What you wanna do now is turn this off and click apply, which will reset your game. IF you're already in-game don't worry, it automatically pulls up exactly where you left off.

Now you should see these models instead of the default original models, which includes different styled guns, different NPC textures, including scientists, barney, HECU marines, etc. If you don't see this then you must have done something wrong.

I honestly like the original models better but you're entitled to your opinion.