The integer variable sec is increased but the new value of the variable isn't automatically shown in the text box, you need to set it. Try the following:
int first;
int sec;
first = int.Parse(textBox2.Text);
sec = int.Parse(txt_netamount.Text);
if (first > 100)
{
sec +=1;
txt_netamount.Text = sec.ToString();
}