There are a few more steps to downloading and implementing this mod, but the developers of the mod have laid it out pretty clearly.
Still, this is essential if you want a cleaner look to your gameplay experience. It’s mainly in the environments and locations that you’ll notice the upgrade, rather than the character models themselves. If you’d like to give Payday 2 a graphical facelift, then you ought to look into this Retexture Project mod.