Form2 frmSec = new Form2();
frmSec.label1.Text = textbox1.Text;

//This is the Form2 constructor, which we have modified
public Form2 (string labText)
{
    InitializeComponent(); //it is important that you put this before the next line of code, else an exception is thrown.
    this.label1 = labText;
}
 
//Now you put this where you call Form2 inside Form1
Form2 frmSec = new Form2(textbox1.Text);

//FORM1 CODE:
//assuming that the Form2 instance inside Form1 is named frmSec
 
//insert into button event handler:
frmSec.ChangeLabel(this.textbox1.Text);
 
//FORM2 code:
public void ChangeLabel(string newText) //make sure this modifier is public
{
    label1.Text = newText;
}