How do update data in one column in sql table

Id   Employee_Id    Cat_Id
1      SSS            1
2      DDD            2
3      FFF            1 
4      GGG            2

DateTime date = SafeConvert.ToDateTime(txtDate.Text);
           Int16 day = Convert.ToInt16(SafeConvert.ToDateTime(txtDate.Text).Day);
           if (day % 2 == 0)
           {
               var empcatone = (from ec in entities.EmployeeCategories where ec.Category_Id == 1 select ec).ToList();
                foreach (var Empcatone in empcatone)
                {

                     Empcatone.Category_Id = 2;



                }
                var empcattwo = _service.GetAllEmployeeCategory().Where(x => x.Category_Id == 2).ToList();
                foreach (var Empcattwo in empcattwo)
                {
                    Empcattwo.Category_Id = 1;

                }

           }
           else if (day % 2 != 0)
           {
               var empcatone = _service.GetAllEmployeeCategory().Where(x => x.Category_Id == 2).ToList();
               foreach (var Empcatone in empcatone)
               {
                   Empcatone.Category_Id = 1;

               }
               var empcattwo = _service.GetAllEmployeeCategory().Where(x => x.Category_Id == 1).ToList();
               foreach (var Empcattwo in empcattwo)
               {
                   Empcattwo.Category_Id = 2;

               }


           }