string ConnectionString = WebConfigurationManager.ConnectionStrings["dbnameConnectionString"].ConnectionString;
SqlConnection myConnection = new SqlConnection(ConnectionString);

myConnection.Open();
try
{
    string qry = "UPDATE customers SET firstname=@firstname WHERE cid=1";
    SqlCommand insertQuery = new SqlCommand(qry, myConnection);
    insertQuery.Parameters.Add(new SqlParameter("@firstname", txtFirstname.Text));
    insertQuery.ExecuteNonQuery();

    myConnection.Close();
}
catch (Exception ee)
{

}

Any suggestions?