c# - Unity Renderer.bounds issue?

I would like to make rend.bounds.max.y be always equal to 20 on collision no matter what, using the script below. But it seems that I can't set it manually and the script has no effect on my GameObject.

Script:

public class ClassKey : MonoBehaviour{

void Start()
   { 
       rend = GetComponent();

   }

void OnCollisionEnter(Collision col)
   {
       //more code
       rend.bounds.max.y = 20;
       //more code
   }
}

What I want to achieve: "Change rend.bounds.max.y to 20 when colliding (and keep the new value forever), and let rend.bounds.max.x and rend.bounds.max.z vary freely when colliding"

The problem: Renderer.bounds is read only... How can I go around this?