Unity 3D How to change skybox?

Question

Dear Stackoverflow community,

I want to change skybox in my game but when i implement script to character and when i put skybox into my main camera skybox is display but I can not see when game progress to have new skybox when game goes on...I mean i can not see night goes to day etc..

The skybox stays the same as before .

This is my skybox settings material:

This is my variable of skybox;

public Material skybox;

This is my script in Update method of character:

UpdateSkybox();

And finally this is method to update skyBox

  void UpdateSkybox()
    {
        skybox.SetFloat("_AtmosphereThickness", Mathf.Repeat(Time.deltaTime / 20, 5f));
    }

I put my material skybox into my character field. Also i add component to my Main camera of material skybox and attached material skybox to it.

p.s. This is what i want to get and this is working on older version of Unity because I found it on some forum.. and make the same.

Maybe my material is not correctly set up but in his game he get night then day etc... I get in mine only this :