A Button, A Boolean and 2 Textures.

First thing I notice is this:

 if (night = true)

Remember that = and == are not the same thing. You are assigning night every time you do this, not testing it.

Hi lebuildman,

I think you want to change the if statement, right now it's usually 1 equal sign instead of two. So it's actually assigning the variable night to be true on that if statement. Also you don't need to change the texture on every frame via Update. You can do it OnMouseDown().

I think you want something like this:

 using UnityEngine;
 using System.Collections;
  
 public class DayNightSelection : MonoBehaviour {
     public static bool night = false;
     public Texture2D NightTexture = Assets/Textures/GUI/Night;
     public Texture2D DayTexture = Assets/Textures/GUI/Day;
     // Update is called once per frame
     void Update () {
     }
     void OnMouseDown() {
         if (night == true)
         {
          night = false;
          guiTexture.texture = DayTexture;
         }
         else
         {
          night = true;
          guiTexture.texture = NightTexture;
         }
     }
 }

Side note: you could remove night bool variable from the code all together. Instead check what the current texture is OnMouseDown() and change to the other texture.