Try this
OracleConnection con = new OracleConnection("Data Source=O10G;User Id=plsql_staj;Password=*****;");
con.Open();
string query = string.Format("UPDATE TEL_REHBERI SET AD ='{0}', SOYAD ='{1}', YAS = '{2}', CEP_NO = '{3}' , EV_NO = '{4}', DOGUMYERI = '{5}' WHERE AD ='{0}' AND SOYAD = '{1}' AND YAS ='{2}' AND CEP_NO = '{3}' AND EV_NO ='{4}' AND DOGUMYERI = '{5}'",txtGnclAd.Text.ToUpper() , txtGnclSoyad.Text.ToUpper(), txtGnclYas.Text.ToUpper() ,txtGnclCep.Text.ToUpper() ,txtGnclEv.Text.ToUpper() , txtGnclDogumYeri.Text.ToUpper());
DataSet ds = new DataSet();
OracleDataAdapter da = new OracleDataAdapter(query, con);
OracleCommand cmd = new OracleCommand(query, con);
bool result = Convert.ToBoolean(cmd.ExecuteNonQuery());
da.Fill(ds);
con.Close();