How do i update student fee allocation in mvc?

[HttpPost]
        public ActionResult Index(CUSTOM_STUDENT_FEEALLOCTION_DATA CUSTOM_STUDENT_FEEALLOCTION_DATAObj)
        {

            var Student_FeeallocationDetail = CUSTOM_STUDENT_FEEALLOCTION_DATAObj.CUSTOM_STUDENT_FEEALLOCATION_DETAIL;
            var SelectStufeedetail = Student_FeeallocationDetail.Where(b => b.SELECTED_STUDENT == true).ToList();

            CUSTOM_STU_DETAIL stuentdetail = new CUSTOM_STU_DETAIL();
            try
            {
                foreach (var studentfeedatal in SelectStufeedetail)
                {
                    if (studentfeedatal.Registration_Id == null)
                    {
                        List<studentregdetail> regIdObj = new List<studentregdetail>();

                        regIdObj = (from r in db.Student_Registration
                                    join ra in db.Student_Re_Admission on r.Registration_Id equals ra.Registration_Id
                                    where r.Academic_Year_Id== studentfeedatal.Academic_Year_Id && r.Class_Id==studentfeedatal.Class_Id
                                    where ra.Promoted==false
                                    
                                    select new StudentRegDetail()
                                    {
                                        Registration_Id = r.Registration_Id,
                                        Admission_No = ra.Admission_No
                                    }).OrderBy(a => a.Registration_Id).ToList();

                        foreach (var regIds in regIdObj)
                        {
                            var fee_set = db.Student_Fee_Allocation.Where(a => a.Fee_Master_Id == studentfeedatal.Fee_Master_Id && a.Registration_Id == regIds.Registration_Id).ToList();
                            if (fee_set.Count == 0)
                            {
                                Student_Fee_Allocation Student_Fee_AlloctionObj = new Student_Fee_Allocation();
                                Student_Fee_AlloctionObj.Last_Updated_Date = DateTime.Now.ToUniversalTime();
                                Student_Fee_AlloctionObj.Last_Updated_By = CurrentUserID;
                                Student_Fee_AlloctionObj.Creation_Date = DateTime.Now.ToUniversalTime();
                                Student_Fee_AlloctionObj.Created_By = CurrentUserID;

                                //Student_Fee_AlloctionObj.Fee_Allocation_Id = studentfeedatal.Fee_Allocation_Id;
                                Student_Fee_AlloctionObj.Academic_Year_Id = studentfeedatal.Academic_Year_Id;
                                Student_Fee_AlloctionObj.Class_Id = studentfeedatal.Class_Id;
                                Student_Fee_AlloctionObj.Admission_No = regIds.Admission_No.ToString();
                                Student_Fee_AlloctionObj.Registration_Id = regIds.Registration_Id;
                                Student_Fee_AlloctionObj.Fee_Master_Id = studentfeedatal.Fee_Master_Id;
                                Student_Fee_AlloctionObj.Fee_Period_Id = studentfeedatal.Fee_Period_Id;
                                Student_Fee_AlloctionObj.Amount = studentfeedatal.Amount;
                                Student_Fee_AlloctionObj.Consession = studentfeedatal.Concession;
                                Student_Fee_AlloctionObj.Final_Amount = studentfeedatal.Final_Amount;
                                Student_Fee_AlloctionObj.Paid_Amount = 0;
                                Student_Fee_AlloctionObj.Balance = studentfeedatal.Final_Amount;
                                Student_Fee_AlloctionObj.Delete_Flag = false;

                                db.Student_Fee_Allocation.AddObject(Student_Fee_AlloctionObj);
                                db.SaveChanges();

                                
                            }
                            else
                            {
                                foreach (var feeset in fee_set)
                                {

                                    db.ObjectStateManager.ChangeObjectState(feeset, EntityState.Modified);
                                    db.SaveChanges();
                                }
                            }
                        }

                        ModelState.AddModelError("", "Fees Allocated Successfully.");
                    }
                    else
                    {
                        var fee_set = db.Student_Fee_Allocation.Where(a => a.Fee_Master_Id == studentfeedatal.Fee_Master_Id && a.Registration_Id == studentfeedatal.Registration_Id).ToList();                        
                        if (fee_set.Count == 0)
                        {
                            Student_Fee_Allocation Student_Fee_AlloctionObj = new Student_Fee_Allocation();
                            Student_Fee_AlloctionObj.Last_Updated_Date = DateTime.Now.ToUniversalTime();
                            Student_Fee_AlloctionObj.Last_Updated_By = CurrentUserID;
                            Student_Fee_AlloctionObj.Creation_Date = DateTime.Now.ToUniversalTime();
                            Student_Fee_AlloctionObj.Created_By = CurrentUserID;

                            //Student_Fee_AlloctionObj.Fee_Allocation_Id = studentfeedatal.Fee_Allocation_Id;
                            Student_Fee_AlloctionObj.Academic_Year_Id = studentfeedatal.Academic_Year_Id;
                            Student_Fee_AlloctionObj.Class_Id = studentfeedatal.Class_Id;
                            Student_Fee_AlloctionObj.Admission_No = studentfeedatal.Admission_No.ToString();
                            Student_Fee_AlloctionObj.Registration_Id = studentfeedatal.Registration_Id;
                            Student_Fee_AlloctionObj.Fee_Master_Id = studentfeedatal.Fee_Master_Id;
                            Student_Fee_AlloctionObj.Fee_Period_Id = studentfeedatal.Fee_Period_Id;
                            Student_Fee_AlloctionObj.Amount = studentfeedatal.Amount;
                            Student_Fee_AlloctionObj.Consession = studentfeedatal.Concession;
                            Student_Fee_AlloctionObj.Final_Amount = studentfeedatal.Final_Amount;
                            Student_Fee_AlloctionObj.Paid_Amount = 0;
                            Student_Fee_AlloctionObj.Balance = studentfeedatal.Final_Amount;
                            Student_Fee_AlloctionObj.Delete_Flag = false;

                            db.Student_Fee_Allocation.AddObject(Student_Fee_AlloctionObj);
                            db.SaveChanges();

                            ModelState.AddModelError("", "Fees Allocated Successfully.");
                        }
                        else
                        {
                            foreach (var feeset in fee_set)
                            {

                                db.ObjectStateManager.ChangeObjectState(feeset, EntityState.Modified);
                                db.SaveChanges();
                            }
                        }
                    }
                }
                FillViewBag_Search();
                return View();
            }
            catch (Exception ex)
            {
                ModelState.AddModelError("", "Error Allocating Fees." + ex.Message.ToString());
                FillViewBag_Search();
                return View();
            }
        }