How to Update in MVC 4.0 using Code first

See more:

MVC4

Hi
I Have model

public class Category
   {
       [Key]
       public int CategoryId { set; get; }
       public string CategoryName { set; get; }
       public int? ParentCategoryId { set; get; }
       public string ParentCategoryName { set; get; }

       [NotMapped]
       public virtual IList<Category> Categories { set; get; }

       public Category()
       {
           Categories = new List<Category>();
       }


   }

and my Update Action

[HttpPost]
      public ActionResult UpdateCategory(Category _category)
      {
          if (_category.CategoryId != 0)
          {
              objCategory = objCategoryContext.CategoryEntries.Where(c => c.CategoryId == _category.CategoryId).FirstOrDefault();

              objCategory.CategoryName = _category.CategoryName;
              objCategoryContext.CategoryEntries.Add(objCategory);
              objCategoryContext.SaveChanges();

          }
          return RedirectToAction("Message");
      }

and my Context

public class CategoryContext : DbContext
    {
        public CategoryContext()
            : base("Name=GunStore")
        {
            Configuration.ProxyCreationEnabled = false;
        }
        public DbSet<Category> CategoryEntries { set; get; }
        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            base.OnModelCreating(modelBuilder);
            modelBuilder.Entity<Category>().ToTable("Category");
        }

    }

When i run this code every time insert new row in my database

Can you tell me where i did the mistake and what i will do ..

Thanks in Advance