如何从WPF的子页面访问父窗口的控件？

你可以做

private void Button_Click_Upload(object sender, RoutedEventArgs e)
{
    ((w)App.Current.MainWindow).textBox_1.Text = "Your Text";
    //or 
    ((w)Window.GetWindow(this)).textBox_1.Text = "Your Text";
}