How to send data from arduino to Temp Controller for setting temp?
You're not writing any data to incomingByte and you're not detecting if data is sent towards the Arduino over serial.

You should change your void loop() into this:

void loop(){

  //Detects if something is sent over serial.
  if(Serial.available() &gt; 0){

    //reads the byte and puts it into the incomingByte variable
    incomingByte = Serial.read();

    //below is your old code
    if (incomingByte == 's') {
      digitalWrite(5, HIGH);
      byte message[] = {0x01, 0x06, 0x00, 0x01, 0x02, 0x58, 0xD8, 0x90} ;
      mySerial.write(message1, sizeof(message1));
    }
  }
}


When you're using the serial monitor to send data towards the Arduino, also make sure the baudrate you use is the same as the Arduino's. In this case you set your baudrate to 9600 Serial.begin(9600).