how to implement migration by changing to identity code firs

Question

I have class department inside container Containerclass as following

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;
using System.Web;

namespace UniversityData.Models
{
    [Table("Department")]
    public partial class Department
    {
        public Department()
            {


            }
        [DatabaseGenerated(DatabaseGeneratedOption.None)]

        [Key]
        public int DepartmentID { get; set; }
        [Required]
        [StringLength(50)]

        public string DepartmentName { get; set; }
        [Required]
        [StringLength(50)]
        public string YearName { get; set; }






    }
}

I need to make DepartmentID field identity then apply migrations in database

how to do that 

my container class as following

 public partial class ContainerClass : DbContext
    {
        public ContainerClass() : base("name=mycon")
        {

        }
        public virtual DbSet<Department> Departments { get; set; }
}

 Table department already created in database but only i need to modify table

to make DepartmentID IDENTITY then make migrations