How to update db by reflection in c#

i want to update a table instance by reflection

here is what i tried

     var type = Assembly.GetExecutingAssembly().GetTypes()
    .FirstOrDefault(t => t.Name == TableName);

             object instance = Activator.CreateInstance(type);

             foreach (var item in dic)
             { 

            PropertyInfo information = type.GetProperties()
.SingleOrDefault(x => x.Name == item.Key);

         information.SetValue(instance, item.Value.ToString(), null); 

              }
             var fx = db.Set(instance.GetType());
                            fx.Add(instance);
            result= db.SaveChanges();

its giving me following exception

   InnerException = {"Violation of PRIMARY KEY constraint
 'PK_primryKeyName. Cannot insert duplicate key in object 'tableName'.
 The duplicate key value is (39).
\r\nThe statement has been terminated."}

its seems Entity framework considers it as an insert rather than an update