instead ofusing my serial monitor use of three leds for indication of ppm leve

Other Hardware LEDs and Multiplexing
1

I have a varible ppm. I can make a grafic with my serial monitor.
Now I want to activate depending the level of measured variablele ppm. three leds.
green orange and red. I dont understand millis . I just use delay for now.
I understand a bit "if (ppm == 0) < and >.

I made a small start.

#include <SoftwareSerial.h>;

SoftwareSerial mySerial(A0, A1); // connect also co2sensor-ardunoUno17 with  Tx-A0,Rx-A1,Vin-5v,GND-GND  ,Old bootloader in my case

byte cmd[9] = {0xFF,0x01,0x86,0x00,0x00,0x00,0x00,0x00,0x79}; 
unsigned char response[9];
unsigned int ppm;

void setup() {
  Serial.begin(9600);
  mySerial.begin(9600);
}

void loop() 
{
  mySerial.write(cmd, 9);
  memset(response, 0, 9);
  mySerial.readBytes(response, 9);
  int i;
  byte crc = 0;
  for (i = 1; i < 8; i++) crc+=response[i];
  crc = 255 - crc;
  crc++;

   if ( !(response[0] == 0xFF && response[1] == 0x86 && response[8] == crc) ) {
    Serial.println("CRC error: " + String(crc) + " / "+ String(response[8]));
  } else {
    unsigned int responseHigh = (unsigned int) response[2];
    unsigned int responseLow = (unsigned int) response[3];
    unsigned int ppm = (256*responseHigh) + responseLow;
    Serial.println(ppm);
  }
  delay(1000);

   if(ppm == 0) {
    Serial.println("PANIK no co2 found!!");
  }

  if(ppm < 500) {
    Serial.println("Oh oh,..<500 we have less co2!!");
  }

  if(ppm > 500) {
    Serial.println("Woohoo 500 or more co2");
  }

    Serial.println("-----------");
    delay(5000);
}
2

I dont understand millis

I assume that you have looked at Using millis() for timing. A beginners guide, Several things at the same time and the BlinkWithoutDelay example in the IDE

Where and when do you want to turn on the LEDs ?

3

Maybe I don't understand the problem.

  if(ppm < 500) {
    Serial.println("Oh oh,..<500 we have less co2!!");

    digitalWrite(ledGreen, LOW);
    digitalWrite(ledYellow, HIGH);
    digitalWrite(ledRed, LOW);
  }

Basically, switch the LEDs off that you want to be off and switch on the one that you want to be on.

As said, I might not understand your problem.