change label.text in children form

The best way is for Form2 to signal an event which Form1 handles and gets the new data via a public property in form2. That way, Form2 need no nothing about Form1, and Form1 need only get teh data if it is actually interested in it.

In the child form:

public partial class frmChild : Form
   {
   // Signal file change happened
   public event EventHandler Changed;

   protected virtual void OnChanged(EventArgs e)
      {
      EventHandler eh = Changed;
      if (eh != null)
         {
         eh(this, e);
         }
      }

   private void DoSomethingToChangeData()
      {
      OnChanged(null);
      }
   }

----- The asign to eh is in case the handler changes between null check
----- and exec.
----- (unlikely, but possible)

----- The null check is to ensure there is a handler. If not, better to
----- ignore it gracefully, than to rely on the thrown exception
----- (NullReferenceException)

In the Parent form:

public frmParent()
    {
    frmChild.Change += new frmChange.ChangeHandler(Changed);
    }

private void ShowChildForm()
    {
    // Link in his event so if it changes, we detect it.
    frmChild fd = new frmChild();
    fd.ShowDialog();
    }

//
// Fired when the data is changed at a lower level.
//
private void Changed(object sender, EventArgs e)
    {
    Form2 f = sender as Form2;
    if (f != null)
        {
        myLabel.Text = f.Data;
        }
    }