i am having trouble merging these two sketches together

Sketch 1:

/*
  AirQuality Demo V1.0.
  connect to A0 to start testing. it will needs about 20s to start 
* By: http://www.seeedstudio.com
*/
#include"AirQuality.h"
#include"Arduino.h"
AirQuality airqualitysensor;
int current_quality =-1;
void setup()
{
    Serial.begin(9600);
    airqualitysensor.init(14);
}
void loop()
{
    current_quality=airqualitysensor.slope();
    if (current_quality >= 0)// if a valid data returned.
    {
        if (current_quality==0)
            Serial.println("High pollution! Force signal active");
        else if (current_quality==1)
            Serial.println("High pollution!");
        else if (current_quality==2)
            Serial.println("Low pollution!");
        else if (current_quality ==3)
            Serial.println("Fresh air");
    }
}
ISR(TIMER1_OVF_vect)
{
	if(airqualitysensor.counter==61)//set 2 seconds as a detected duty
	{

			airqualitysensor.last_vol=airqualitysensor.first_vol;
			airqualitysensor.first_vol=analogRead(A0);
			airqualitysensor.counter=0;
			airqualitysensor.timer_index=1;
			PORTB=PORTB^0x20;
	}
	else
	{
		airqualitysensor.counter++;
	}
}

Sketch 2:

#include <LiquidCrystal.h>  
int Contrast=20;
 LiquidCrystal lcd(12, 11, 5, 4, 3, 2);   

 void setup()
 { 
    analogWrite(6,Contrast);
     lcd.begin(16, 2);
  }  
     void loop()
 {  
     lcd.setCursor(0, 0);
     lcd.print("Air Quality Sensor");
    
    lcd.setCursor(0, 1);
     lcd.print("Open Serial Monitor");
}

I have tried many ways and still having trouble. I would really like to have this project done soon.

You have a sketch that does something. You did not bother saying what it actually does.

You have another sketch that does something. You did not bother saying what it actually does.

You want to create another sketch that does something. You did not bother saying what you want the resulting program to do. You did not bother posting your attempt to create that program. Nor did you explain what the attempt actually does (if it even compiles) or how that differed from what you actually wanted it to do.

I would really like to have this project done soon.

Best get busy, then.

Merged fine for me:

#include"AirQuality.h"

AirQuality airqualitysensor;
int current_quality = -1;

#include <LiquidCrystal.h>
int Contrast = 20;
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

void setup()
{
  Serial.begin(9600);
  airqualitysensor.init(14);
  analogWrite(6, Contrast);
  lcd.begin(16, 2);
}

void loop()
{
  current_quality = airqualitysensor.slope();
  if (current_quality >= 0)// if a valid data returned.
  {
    if (current_quality == 0)
      Serial.println("High pollution! Force signal active");
    else if (current_quality == 1)
      Serial.println("High pollution!");
    else if (current_quality == 2)
      Serial.println("Low pollution!");
    else if (current_quality == 3)
      Serial.println("Fresh air");
  }
  
  lcd.setCursor(0, 0);
  lcd.print("Air Quality Sensor");

  lcd.setCursor(0, 1);
  lcd.print("Open Serial Monitor");
}

ISR(TIMER1_OVF_vect)
{
  if (airqualitysensor.counter == 61) //set 2 seconds as a detected duty
  {
    airqualitysensor.last_vol = airqualitysensor.first_vol;
    airqualitysensor.first_vol = analogRead(A0);
    airqualitysensor.counter = 0;
    airqualitysensor.timer_index = 1;
    PORTB = PORTB ^ 0x20;
  }
  else
  {
    airqualitysensor.counter++;
  }
}

johnwasser:
Merged fine for me:

#include"AirQuality.h"

AirQuality airqualitysensor;
int current_quality = -1;

#include <LiquidCrystal.h>
int Contrast = 20;
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

void setup()
{
  Serial.begin(9600);
  airqualitysensor.init(14);
  analogWrite(6, Contrast);
  lcd.begin(16, 2);
}

void loop()
{
  current_quality = airqualitysensor.slope();
  if (current_quality >= 0)// if a valid data returned.
  {
    if (current_quality == 0)
      Serial.println(“High pollution! Force signal active”);
    else if (current_quality == 1)
      Serial.println(“High pollution!”);
    else if (current_quality == 2)
      Serial.println(“Low pollution!”);
    else if (current_quality == 3)
      Serial.println(“Fresh air”);
  }
 
  lcd.setCursor(0, 0);
  lcd.print(“Air Quality Sensor”);

lcd.setCursor(0, 1);
  lcd.print(“Open Serial Monitor”);
}

ISR(TIMER1_OVF_vect)
{
  if (airqualitysensor.counter == 61) //set 2 seconds as a detected duty
  {
    airqualitysensor.last_vol = airqualitysensor.first_vol;
    airqualitysensor.first_vol = analogRead(A0);
    airqualitysensor.counter = 0;
    airqualitysensor.timer_index = 1;
    PORTB = PORTB ^ 0x20;
  }
  else
  {
    airqualitysensor.counter++;
  }
}

I tried later and got this

/*
Air Quality Sensor
*/
#include"AirQuality.h"
#include"Arduino.h"
#include <LiquidCrystal.h>  
int Contrast=20;
 LiquidCrystal lcd(12, 11, 5, 4, 3, 2);   
AirQuality airqualitysensor;
int current_quality =-1;
void setup()
{
    Serial.begin(9600);
    airqualitysensor.init(14);
    analogWrite(6,Contrast);
     lcd.begin(16, 2);
}
void loop()
{
    current_quality=airqualitysensor.slope();
    if (current_quality >= 0)// if a valid data returned.
    {
        if (current_quality==0)
            Serial.println("High pollution! Force signal active");
        else if (current_quality==1)
            Serial.println("High pollution!");
        else if (current_quality==2)
            Serial.println("Low pollution!");
        else if (current_quality ==3)
            Serial.println("Fresh air");
     lcd.setCursor(0, 0);
     lcd.print("Air Quality");
    
    lcd.setCursor(0, 1);
     lcd.print("Sensor");
    }
}
ISR(TIMER1_OVF_vect)
{
	if(airqualitysensor.counter==61)//set 2 seconds as a detected duty
	{

			airqualitysensor.last_vol=airqualitysensor.first_vol;
			airqualitysensor.first_vol=analogRead(A0);
			airqualitysensor.counter=0;
			airqualitysensor.timer_index=1;
			PORTB=PORTB^0x20;
	}
	else
	{
		airqualitysensor.counter++;
	}
}

it seems to work but the numbers are very different

what would be more accurate? its an air quality sensor

More accurate than what? What do you have? What do you want it to do? What accuracy are you expecting?