How to edit selected row of DataGrid and add the new row to DataGrid

MainWindow.xaml

<Window x:Class="WpfEx.Views.MainWindow"
       xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
       xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
       xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
       xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
       xmlns:local="clr-namespace:WpfEx"
       mc:Ignorable="d">
  <StackPanel>

<DataGrid ItemsSource="{Binding LstStudents}" SelectedItem="{Binding SelectedStudent}" AutoGenerateColumns="False" Height="200" Width="200" HorizontalAlignment="Left">
           <DataGrid.Columns>
               <DataGridTextColumn Header="Name" Binding="{Binding Name}"/>
               <DataGridTextColumn Header="Roll" Binding="{Binding Roll}"/>
           </DataGrid.Columns>
       </DataGrid>
       <Button Content="Edit Selected row" Command="{Binding EditSelectedRowCommand}" HorizontalAlignment="Left" Width="150"/>
       <Button Content="Add row" Command="{Binding AddRowCommand}" HorizontalAlignment="Left" Width="150"/>
</StackPanel>
</Window>

MainWindow.xaml.cs

 public partial class MainWindow : Window
   {
     
       public MainWindow()
       {
           InitializeComponent();
             DataContext= new MainWindowViewModel(); 
       }

   }

MainWindowViewModel.cs

 public class MainWindowViewModel : BindableBase
   {
       public MainWindowViewModel()
       {
           EditSelectedRowCommand = new DelegateCommand(EditRow);
           AddRowCommand = new DelegateCommand(AddRow);

    LstStudents =new ObservableCollection<Student>
           {
               new Student
               {
                   Name="ABC",
                   Roll=1
               },
               new Student
               {
                   Name="DEF",
                   Roll=2
               }
           };         
       }

       private void AddRow()
       {
           var studentToAdd = new Student();
           var addWindow = new Add(studentToAdd);
         
           var window = new Window
           {
               Title = "Add Student",
               Content = addWindow
           };

           addWindow.AddStudent.Click += (s, e) =>
     {
      LstStudents.Add(studentToAdd);
     window.Close();
    };
   
           window.Show();
       }

       private void EditRow()
       {
    var editWindow = new Window
           {
               Title="Edit Selected Student",
               Content = new Edit(SelectedStudent)
           };
           editWindow.Show();
       }

       private ObservableCollection<Student> _lstStudents =
    new ObservableCollection<Student>();

       public ObservableCollection<Student> LstStudents
       {
           get { return _lstStudents; }
           set { SetProperty(ref _lstStudents, value); }
       }

       private Student _selectedStudent=new Student();

       public Student SelectedStudent
       {
           get { return _selectedStudent; }
           set { SetProperty(ref _selectedStudent ,value); }
       }

       public ICommand EditSelectedRowCommand { get; set; }
       public ICommand AddRowCommand { get; set; }

}

Student.cs

public partial class Student
   {

       public int Roll { get; set; }
       public string Name { get; set; }
   }

Add.xaml

<UserControl x:Class=" WpfEx.Views.Add"
            xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
            xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
            xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
            xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
            xmlns:local="clr-namespace: WpfEx.Views"
            mc:Ignorable="d" >
   <Grid>
       <Grid.RowDefinitions>
           <RowDefinition Height="40"/>
           <RowDefinition Height="40"/>
           <RowDefinition Height="40"/>
       </Grid.RowDefinitions>
       <Grid.ColumnDefinitions>
           <ColumnDefinition Width="100"/>
           <ColumnDefinition Width="*"/>
       </Grid.ColumnDefinitions>
       <TextBlock Text="Name : "/>
       <TextBox Width="200" Text="{Binding Name}" Grid.Column="1" Margin="5" HorizontalAlignment="Left"/>
     
       <TextBlock Text="Roll : " Grid.Row="1"/>
       <TextBox Width="200" Text="{Binding Roll}"  Grid.Column="1" Grid.Row="1" Margin="5"  HorizontalAlignment="Left"/>
       <Button Content="Add" x:Name="AddStudent" Grid.Column="1" Grid.Row="2" HorizontalAlignment="Right" x:FieldModifier="public" Margin="5" Width="150"/>

   </Grid>
</UserControl>

Add.xaml.cs

public partial class Add : UserControl
   {
       public Add(Student student)
       {
           InitializeComponent();
           DataContext = student;
       }
   }

Edit.xaml

<UserControl x:Class=" WpfEx.Views.Edit"
            xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
            xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
            xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
            xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
            xmlns:local="clr-namespace: WpfEx.Views"
            mc:Ignorable="d"
            d:DesignHeight="450" d:DesignWidth="800">
   <Grid>
       <Grid.RowDefinitions>
           <RowDefinition Height="40"/>
           <RowDefinition Height="40"/>
       </Grid.RowDefinitions>
       <Grid.ColumnDefinitions>
           <ColumnDefinition Width="100"/>
           <ColumnDefinition Width="*"/>
       </Grid.ColumnDefinitions>
       <TextBlock Text="Name : "/>
       <TextBox Width="200" Text="{Binding Name}" Grid.Column="1" Margin="5" HorizontalAlignment="Left"/>
     
       <TextBlock Text="Roll : " Grid.Row="1"/>
       <TextBox Width="200" Text="{Binding Roll}"  Grid.Column="1" Grid.Row="1" Margin="5"  HorizontalAlignment="Left"/>
   </Grid>
</UserControl>

Edit.xaml.cs

public partial class Edit : UserControl
   {
       public Edit(Student student)
       {
           InitializeComponent();
           DataContext = student;
       }
   }