I want to update the leftdays, Row by Row .not all at once.

Question

I want to Update the Leftdays Column Row by Row but the Statement which  i am using for Update .its updates the whole Column with same value .. as datareader loop calculates the different value while looping ... how i can make work properly ..please comment the Code .. 

private void button3_Click(object sender, EventArgs e)

{

myconnection.Open();

displayData();

string selectSql = "select * from employTable2";

SqlCommand com = new SqlCommand(selectSql, myconnection);

using (SqlDataReader read = com.ExecuteReader())

{

while (read.Read())

{

string first1 = read["salarydate"].ToString();

string Second1 = read["expiredate"].ToString();

TimeSpan t1 = Convert.ToDateTime(Second1) - DateTime.Now;

int daysleft = t1.Days;

SqlConnection myconnection1 = new SqlConnection(@"Data Source=.\SQLEXPRESS;AttachDbFilename=c:\documents and settings\muhammad\my documents\visual studio 2010\Projects\ProjectClock\ProjectClock\ClockDB.mdf;Integrated Security=True;User Instance=True;");

myconnection1.Open();

SqlCommand cmd1 = new SqlCommand("UPDATE employTable2 SET daysleft='" + daysleft.ToString() + "'", myconnection1);

cmd1.ExecuteNonQuery();

myconnection1.Close();

MessageBox.Show(t1.Days +"", "Days Left");

}

myconnection.Close();

}

}