C#

using NamespaceTest;

namespace WindowsFormsApplication3
{
    
    public partial class Form1 : Form
    {
       
        public Form1()
        {
            InitializeComponent();
        }

        public string LabelText
        {
            get { return Label1.Text; }
            set { Label1.Text = value; }
        }
        private void button1_Click(object sender, EventArgs e)
        {
            MessageBox.Show("Button was pressed");
            ClassTEST d = new ClassTEST();
          d.Changelabel();
            
        }
..

using System;
using WindowsFormsApplication3;

namespace NamespaceTest
{
    public class ClassTEST
    {
        // private Form1 form1;
        //public static ClassTEST
        //{  
        //form1 = form2;

        //}
        public void Changelabel()
        {


            Form1 form = new Form1();
            form.LabelText = "text";
        }


    }

public void Changelabel(Form1 form)
{
  form.LabelText = "text";
}

private void button1_Click(object sender, EventArgs e)
{
  new ClassTEST().ChangeLabel(this); // "this" is a keyword that identifies the current instance.
}

Nigel Mackay wrote:

With which event handler?

Bastard Programmer from Hell
If you can't read my code, try converting it here[^]