How do you change the text in the Titlebar in Windows Forms?

You can change the text in the titlebar in Windows Forms by using the Text property.

For C#

// This class is added to the namespace containing the Form1 class.
class MainApplication
{
   public static void Main()
   {
      // Instantiate a new instance of Form1.
      Form1 f1 = new Form1();

      // Display a messagebox. This shows the application
      // is running, yet there is nothing shown to the user.
      // This is the point at which you customize your form.
      System.Windows.Forms.MessageBox.Show("The application "
         + "is running now, but no forms have been shown.");

      // Customize the form.
      f1.Text = "Running Form";

      // Show the instance of the form modally.
      f1.ShowDialog();
   }
}

All the answers that include creating an new object from Form class are absolutely creating new form. But you can use Text property of ActiveForm subclass in Form class. For example:

        public Form1()
    {
        InitializeComponent();
        Form1.ActiveForm.Text = "Your Title";
    }

For changing the Title of a form at runtime we can code as below

public partial class FormMain : Form
{
    public FormMain()
    {
        InitializeComponent();
        this.Text = "This Is My Title";
    }
}