How can I rotate Cubemap?

I am currently working on some reflection shaders in Unity. And I am totally newbie about shaders. I found an article about cubemap rotation. I' ve done some implementation but it seems doesn't work properly. I rotated cubemap in vertex shader with only rotate normal. How can I achive this effect?

  float3 RotateAroundYInDegrees (float3 vertex, float degrees)
       {
          float alpha = degrees * UNITY_PI / 180.0;
          float sina, cosa;
          sincos(alpha / 2, sina, cosa);
          float3x3 m = float3x3(cosa, 0, sina, 0, 1, 0, -sina, 0, cosa);
         float3 r = float3(mul(m, vertex.xyz) ).rgb;
         return r;
     }
     void vert (inout appdata_full v, out Input o) {
         UNITY_INITIALIZE_OUTPUT(Input,o);
         v.normal = RotateAroundYInDegrees(v.normal, _Rotate);
       }