How to fix this cube ?

You would need the following script. I dont remember where I got it from, but it makes the Blender models right side up for me. Just make sure to put it in the "Editor" folder and whatnot.

 using UnityEngine;
 using System.Collections;
 using UnityEditor;
 using System.IO;
 using System.Linq;
 
 
 public class BlenderAssetProcessor : AssetPostprocessor {
     public void OnPostprocessModel(GameObject obj  )
     {
 
         //only perform corrections with blender files
 
         ModelImporter importer = assetImporter as ModelImporter;
         if (Path.GetExtension(importer.assetPath) == ".blend")
         {
             RotateObject(obj.transform);
         }
 
         //Don't know why we need this...
         //Fixes wrong parent rotation
         obj.transform.rotation = Quaternion.identity;
     }
 
     //recursively rotate a object tree individualy
     private void RotateObject(Transform obj  )
     {
         Vector3 objRotation = obj.eulerAngles;
         objRotation.x += 90f;
         obj.eulerAngles = objRotation;
 
         //if a meshFilter is attached, we rotate the vertex mesh data
         MeshFilter meshFilter = obj.GetComponent(typeof(MeshFilter)) as MeshFilter;
         if (meshFilter)
         {
             RotateMesh(meshFilter.sharedMesh);
         }
 
         //do this too for all our children
         //Casting is done to get rid of implicit downcast errors
         foreach (Transform child in obj)
         {
             RotateObject(child);
         }
     }
 
     //"rotate" the mesh data
     private void RotateMesh(Mesh mesh)
     {
         int index = 0;
 
         //switch all vertex z values with y values
         Vector3[] vertices = mesh.vertices;
         for (index = 0; index < vertices.Length; index++)
         {
             vertices[index] = new Vector3(vertices[index].x, vertices[index].z, vertices[index].y);
         }
         mesh.vertices = vertices;
 
         //for each submesh, we invert the order of vertices for all triangles
         //for some reason changing the vertex positions flips all the normals???
         for (int submesh = 0; submesh < mesh.subMeshCount; submesh++)
         {
             int[] triangles = mesh.GetTriangles(submesh);
             for (index = 0; index < triangles.Length; index += 3)
             {
                 int intermediate = triangles[index];
                 triangles[index] = triangles[index + 2];
                 triangles[index + 2] = intermediate;
             }
             mesh.SetTriangles(triangles, submesh);
         }
 
         //recalculate other relevant mesh data
         mesh.RecalculateNormals();
         mesh.RecalculateBounds();
     }
 }