How Do I Recognize & Edit The Specific Cell In Datagrid?

MainWindow.xaml.cs
------------------

using MVVM_WPFSample.ViewModel;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Data;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;


namespace MVVM_WPFSample
{
///

/// Interaction logic for MainWindow.xaml
///

public partial class MainWindow : Window
{

private CustomerMainViewModel customerviewmodel=null;

public MainWindow()
{
InitializeComponent();
}

Country country = new Country();

public IList<country> Countries
{
get;
private set;
}



private void Window_Load(object sender, RoutedEventArgs e)
{
customerviewmodel = new CustomerMainViewModel();
//this.DataContext = customerviewmodel;

this.Countries = new Collection<country>()
{
new Country(){CountryName="India" ,
States = new Collection<states>()
{
new States(){ StateName = "AndhraPradesh" },
new States(){ StateName = "Bihar" },
new States(){ StateName = "Chandigarh" }
}},

new Country(){CountryName="USA" ,
States = new Collection<states>()
{
new States(){ StateName = "London" },
new States(){ StateName = "Birmingham" },
new States(){ StateName = "Glasgow" }
}},

new Country(){CountryName="Austraila" ,
States = new Collection<states>()
{
new States(){ StateName = "Sedney" },
new States(){ StateName = "Melbourn" },
new States(){ StateName = "Perth" }
}},

};
countryddl.ItemsSource = Countries;
//stateddl.ItemsSource = Countries[0].States;

}

private void Countries_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
if (countryddl.SelectedIndex == 0)
stateddl.ItemsSource = Countries[0].States;
if (countryddl.SelectedIndex == 1)
stateddl.ItemsSource = Countries[1].States;
if (countryddl.SelectedIndex == 2)
stateddl.ItemsSource = Countries[2].States;

}
List<customermainviewmodel> customer = new List<customermainviewmodel>();
private void Save_Click(object sender, RoutedEventArgs e)
{


customerviewmodel.NameTextField = tbName.Text;
customerviewmodel.AddressTextField = tbAddress.Text;
customerviewmodel.CountryField = countryddl.Text;
customerviewmodel.StateField = stateddl.Text;
customerviewmodel.Product = customerviewmodel.Product1 + "," + customerviewmodel.Product2;
customer.Add(customerviewmodel);
dataGrid1.Items.Add(customer);

MessageBox.Show("Data Successfully Saved", " MessageBox", MessageBoxButton.OK, MessageBoxImage.Asterisk);

clearValues();

}

private void MaleRadio_CheckedEvent(object sender, RoutedEventArgs e)
{
customerviewmodel.GenderField = "Male";
}

private void FemaleRadio_CheckedEvent(object sender, RoutedEventArgs e)
{
customerviewmodel.GenderField = "Female";
}




private void ProductTV_CheckedEvent(object sender, RoutedEventArgs e)