how to focus texbox by pressing backspace key in wpf

On your window you should add the PreviewKeyUp event.
if the key is backspace, set handled to true.
Call RequestFocusChange()

PreviewKeyUp=Window_PreviewKeyUp

private void Window_PreviewKeyUp(object sender, System.Windows.Input.KeyEventArgs e)
{
   if (e.Key==System.Windows.Input.Key.Back)
    {
       e.Handled = true;
       RequestFocusChange(FocusNavigationDirection.Previous);
    }
}

private static bool RequestFocusChange(FocusNavigationDirection direction)
{
  var focused = Keyboard.FocusedElement as UIElement;
  if (focused != null) return focused.MoveFocus(new TraversalRequest(direction));
  return false;
}