How to update UI controls from another thread in ASP.NET?

Question

How to update UI controls(ex. list box) from another thread in ASP.NET? From the below code I am expecting to update listbox once performLongTask1() and performLongTask2() completes execution and also should update in the order of finishing task.

protected void Button1_Click(object sender, EventArgs e)
{
            Thread t1 = new Thread(performLongTask1);
            Thread t2 = new Thread(performLongTask2);
            t1.Start();
            t2.Start();
            ListBox1.Items.Add("LongTask1 Started");
            ListBox1.Items.Add("LongTask2 Started");
}
private  void performLongTask1()
{
            Thread.Sleep(5000);
           //ListBox1.Items.Add("LongTask1 Completed");
}
private  void performLongTask2()
{
           Thread.Sleep(10000);
           //ListBox1.Items.Add("LongTask2 Completed");
}