How do I invert normals of a sphere???

Here is an editor script that will produce an inverted sphere. Put it in your Assets/Editor folder of your project. Then GameObject > Create Other > Inverted Sphere...

 using UnityEngine;
 using UnityEditor;
 
 public class InvertedSphere : EditorWindow {
     private string st = "1.0";
  
     [MenuItem("GameObject/Create Other/Inverted Sphere...")]
     public static void ShowWindow() {
         EditorWindow.GetWindow(typeof(InvertedSphere));
     }
  
     public void OnGUI() {
         GUILayout.Label("Enter sphere size:");
         st = GUILayout.TextField (st);
         
         float f;
         if (!float.TryParse (st, out f))
             f = 1.0f;
         if (GUILayout.Button("Create Inverted Sphere")) {
             CreateInvertedSphere(f);
         }
     }
     
     private void CreateInvertedSphere(float size) {
         GameObject go = GameObject.CreatePrimitive(PrimitiveType.Sphere);
         MeshFilter mf = go.GetComponent<MeshFilter>();
         Mesh mesh = mf.sharedMesh;
         
         GameObject goNew = new GameObject();    
         goNew.name = "Inverted Sphere";
         MeshFilter mfNew = goNew.AddComponent<MeshFilter>();
         mfNew.sharedMesh = new Mesh();
 
         
         //Scale the vertices;
         Vector3[] vertices = mesh.vertices;
         for (int i = 0; i < vertices.Length; i++)
             vertices[i] = vertices[i] * size;
         mfNew.sharedMesh.vertices = vertices;
         
         // Reverse the triangles
         int[] triangles = mesh.triangles;
         for (int i = 0; i < triangles.Length; i += 3) {
             int t = triangles[i];
             triangles[i] = triangles[i+2];
             triangles[i+2] = t;
         }
         mfNew.sharedMesh.triangles = triangles;
         
         // Reverse the normals;
         Vector3[] normals = mesh.normals;
         for (int i = 0; i < normals.Length; i++)
             normals[i] = -normals[i];
         mfNew.sharedMesh.normals = normals;
         
         
         mfNew.sharedMesh.uv = mesh.uv;
         mfNew.sharedMesh.uv2 = mesh.uv2;
         mfNew.sharedMesh.RecalculateBounds();
         
         // Add the same material that the original sphere used
         MeshRenderer mr = goNew.AddComponent<MeshRenderer>();
         mr.sharedMaterial = go.renderer.sharedMaterial;
         
         DestroyImmediate(go);
     }
 }

Here is a shader for back face rendering.

 Shader "Backfaced Bumped Diffuse" {
 Properties {
     _Color ("Main Color", Color) = (1,1,1,1)
     _MainTex ("Base (RGB)", 2D) = "white" {}
     _BumpMap ("Normalmap", 2D) = "bump" {}
 }
 
 SubShader {
     Tags { "RenderType"="Opaque" }
     LOD 300
     Cull Off
 
 CGPROGRAM
 #pragma surface surf Lambert
 
 sampler2D _MainTex;
 sampler2D _BumpMap;
 fixed4 _Color;
 
 struct Input {
     float2 uv_MainTex;
     float2 uv_BumpMap;
 };
 
 void surf (Input IN, inout SurfaceOutput o) {
     fixed4 c = tex2D(_MainTex, IN.uv_MainTex) * _Color;
     o.Albedo = c.rgb;
     o.Alpha = c.a;
     o.Normal = UnpackNormal(tex2D(_BumpMap, IN.uv_BumpMap));
 }
 ENDCG  
 }
 
 FallBack "Diffuse"
 }

To Use: In Unity Editor Right Click > Create > Shader, Name it Backface Bumped Diffuse, Open the shader with monodevelop and paste the code above, Now change your materials that are having issues to "Backface Bumped Diffuse".