try this, refer in line comments
Form2
public partial class Form2 : Form
{
public string SumValue { get; set; }
private void textBox1_TextChanged(object sender, EventArgs e)
{
SumValue = textBox1.Text;
}
Form1
public partial class Form1 : Form
{
private void button1_Click(object sender, EventArgs e)
{
Form2 objForm2 = new Form2();
objForm2.ShowDialog();
this.textBox1.Text = objForm2.SumValue;
}