How do I convert it to switch case?

const int red = 5;
const int green = 6;
const int blue = 9;

int lastState = LOW;
int r = 0; //red
int g = 0; //green
int b = 0; //blue
int dir = 0; 
int led = 11;
int b1, b2; // buttons

void setup() {
  pinMode(2, INPUT);
  pinMode(3, INPUT);
  pinMode(red, OUTPUT);
  pinMode(green, OUTPUT);
  pinMode(blue, OUTPUT);
  for (int i = 11; i <= 18; i++) {
    pinMode(i, OUTPUT);
  }
}

void loop() {
  b1 = digitalRead(2);
  b2 = digitalRead(3);

  if (b1 == HIGH) {
    dir = 1;
  } else {
    dir = 0;
  }
  digitalWrite(led, LOW);
  if (dir == 1 && led < 18) {
    led++;
  } else if (dir == 1 && led == 18) {
    led = 11;
  }
  digitalWrite(led, HIGH);
  delay(200);

  if(b2 == HIGH && lastState == LOW) {
    function(led, 1);
  }
  if (b2 == HIGH && lastState == HIGH) {
    function(led, 0);
  }
  lastState = b2;
}

void function(int pin, int sw) {

  if(pin == 11 && sw == 1) {
    digitalWrite(red, HIGH);
  } else if (pin == 11 && sw == 0) {
    digitalWrite(red, LOW);
  } 
  if (pin == 12) {
     if (pin == 12 && sw == 1 || sw == 0){
       r = r +5;
     }else if (r == 255){
      r = 0;
     } 
    analogWrite(red, r); 
  }
  if (pin == 13 && sw == 0) {
    digitalWrite(green, HIGH);
  } else if (pin == 13 && sw == 1) {
    digitalWrite(green, LOW);
  } if (pin == 14) {
    if (pin == 14 && sw == 1 || sw == 0) {
      g = g+5; 
    } else if (g = 255) {
      g = 0;
    }
    analogWrite(green, g);
  }
  if(pin == 15 && sw == 0) {
    digitalWrite(blue, HIGH);
  } else if (pin == 15 && sw == 1) {
    digitalWrite(blue, LOW);
  } if (pin == 16) {
    if (pin == 16 && sw == 1 || sw == 0) {
      b = b+5;
    } else if (b == 255) {
      b = 0;
    }
    analogWrite(blue, b);
  }
  if(pin == 17) {
    analogWrite(red, 255);
    analogWrite(green, 255);
    analogWrite(blue, 255);
  } else if (pin == 18) {
    analogWrite(red, 0);
    analogWrite(green, 0);
    analogWrite(blue, 0);
  }
}

Hi I just want to make it shorter is it possible to make it shorter by using switch case in void function() {
?

Why don't you give it a try?

switch(pin) {
   case 11:
       ....
   break;
   case 12:
     .....
    break;
   .....
}

Would be good to declare pin names....