You need to run two SQL statements:
SqlCommand com1 = new SqlCommand("SELECT * FROM EMP_INFO", con);
SqlDataReader r = com1.ExecuteReader();
int i = 1;
while (r.Read())
{
string name = (string) r["name"];
string address = (string) r["address"];
string phone = (string) r["phn_no"];
SqlCommand com2 = new SqlCommand(string.Format("UPDATE EMP_INFO SET empid=EMP{0} WHERE name=@N AND address=@A AND phn_np=@P", i.ToString("D4")), con);
com2.Parameters.AddwithValue("@N",name);
com2.Parameters.AddwithValue("@A",address);
com2.Parameters.AddwithValue("@P",phone);
com2.ExecuteNonQuery();
i++;
}