Counter UP down

int a=2;

int b=3;
int c=4;
int d=6;
int dp=5;
int e=7;
int f=8;
int g=9;
int btn1=13;
int btn2=12;
int counter=0;
int buttonUpState=0;
int lastButtonUpState=0;
int buttonDownState=0;
int lastButtonDownState=0;

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

  pinMode(a,OUTPUT);
  pinMode(b,OUTPUT);
  pinMode(c,OUTPUT);
  pinMode(d,OUTPUT);
  pinMode(e,OUTPUT);
  pinMode(f,OUTPUT);
  pinMode(g,OUTPUT);
  pinMode(dp,OUTPUT);

  digitalWrite(dp,HIGH);
}

void loop()
{
  buttonUpState=digitalRead(btn1);
  buttonDownState=digitalRead(btn2);

  if(buttonUpState!=lastButtonUpState)
  {
    if(buttonUpState==HIGH)
    {
      if(counter==9)
      {
        counter=-1;
      }
      counter++;
      Serial.println(counter);
      changeNumber(counter);
      delay(300);
    }
    else
    {
        Serial.println("OFF");
    }
   delay(50);
  }


  if(buttonDownState!=lastButtonDownState)
  {
    if(buttonDownState==HIGH)
    {
      if(counter == 0)
      {
        counter = 10;
      }
      counter--;
      Serial.println(counter);
      changeNumber(counter);
      delay(300);
    }
    else
    {
        Serial.println("OFF");
    }
   delay(50);
  }
  changeNumber(counter);
}





void changeNumber(int buttonPress)
{
  switch (buttonPress)
  {
    //number 0
    case 0:
      digitalWrite(a,LOW);
      digitalWrite(b,LOW);
      digitalWrite(c,LOW);
      digitalWrite(d,LOW);
      digitalWrite(e,LOW);
      digitalWrite(f,LOW);
      digitalWrite(g,HIGH);
      break;
    //number 1
    case 1:
      digitalWrite(a,HIGH);
      digitalWrite(b,LOW);
      digitalWrite(c,LOW);
      digitalWrite(d,HIGH);
      digitalWrite(e,HIGH);
      digitalWrite(f,HIGH);
      digitalWrite(g,HIGH);
      break;
    //number 2
    case 2:
      digitalWrite(a,LOW);
      digitalWrite(b,LOW);
      digitalWrite(c,HIGH);
      digitalWrite(d,LOW);
      digitalWrite(e,LOW);
      digitalWrite(f,HIGH);
      digitalWrite(g,LOW);
      break;
    //number 3
    case 3:
      digitalWrite(a,LOW);
      digitalWrite(b,LOW);
      digitalWrite(c,LOW);
      digitalWrite(d,LOW);
      digitalWrite(e,HIGH);
      digitalWrite(f,HIGH);
      digitalWrite(g,LOW);
      break;
    //number 4
    case 4:
      digitalWrite(a,HIGH);
      digitalWrite(b,LOW);
      digitalWrite(c,LOW);
      digitalWrite(d,HIGH);
      digitalWrite(e,HIGH);
      digitalWrite(f,LOW);
      digitalWrite(g,LOW);
      break;
    //number 5
    case 5:
      digitalWrite(a,LOW);
      digitalWrite(b,HIGH);
      digitalWrite(c,LOW);
      digitalWrite(d,LOW);
      digitalWrite(e,HIGH);
      digitalWrite(f,LOW);
      digitalWrite(g,LOW);
      break;
    //number 6
    case 6:
      digitalWrite(a,LOW);