RFID multiple tags?

Using Arduino Programming Questions
1

i'm having trouble with writing code, i'm trying to self teach myself, but its not good well. have hacked bits of different code together,to make my alarm system project but i cant figure out how to add more then one RFID tag to the code. is it possible that someone can give us a hand trying to sort it thanks.

this is what have.

/*
 Typical pin layout used:
   -----------------------------------------------------------------------------------------
               MFRC522      Arduino       Arduino    
               Reader/PCD   Uno/101       Nano v3    
   Signal      Pin          Pin           Pin       
   -----------------------------------------------------------------------------------------
   RST/Reset   RST          9             D9      orange   
   SPI SS      SDA(SS)      10            D10     yellow   
   SPI MOSI    MOSI         11 / ICSP-4   D11     green  
   SPI MISO    MISO         12 / ICSP-1   D12     blue  
   SPI SCK     SCK          13 / ICSP-3   D13     purple   
*/
#include <SPI.h>
#include <MFRC522.h>


#define SS_PIN 10
#define RST_PIN 9
MFRC522 mfrc522(SS_PIN, RST_PIN);   // Create MFRC522 instance.
int RELAY = 8;
int REDLED = 7;
int GREENLED = 6;
int BUZZER = 5;

void setup()
{
  Serial.begin(9600);   // Initiate a serial communication
  SPI.begin();      // Initiate  SPI bus
  mfrc522.PCD_Init();   // Initiate MFRC522
  Serial.println("SHED DOOR.");
  Serial.println();
  pinMode (RELAY, OUTPUT);
  pinMode (REDLED, OUTPUT);
  pinMode (GREENLED, OUTPUT);
  pinMode (BUZZER, OUTPUT);
}


void loop()
{
  // Look for new cards
  if ( ! mfrc522.PICC_IsNewCardPresent())
  {
    return;
  }
  // Select one of the cards
  if ( ! mfrc522.PICC_ReadCardSerial())
  {
    return;
  }
  //Show UID on serial monitor
  Serial.print("UID tag :");
  String content = "";
  byte letter;
  for (byte i = 0; i < mfrc522.uid.size; i++)
  {
    Serial.print(mfrc522.uid.uidByte[i] < 0x10 ? " 0" : " ");
    Serial.print(mfrc522.uid.uidByte[i], HEX);
    content.concat(String(mfrc522.uid.uidByte[i] < 0x10 ? " 0" : " "));
    content.concat(String(mfrc522.uid.uidByte[i], HEX));
  }
  Serial.println();
  Serial.print("Message : ");
  content.toUpperCase();
  if (content.substring(1) == "85 A5 B0 56") //change here the UID of the card/cards that you want to give access
   {
    Serial.println("Authorized access");
    Serial.println("Welcome Mr Hughes");
    digitalWrite (BUZZER, HIGH);
    delay(500);
    digitalWrite (BUZZER, LOW);
    digitalWrite (RELAY, HIGH);
    digitalWrite (GREENLED, HIGH);
    delay(10000);
    digitalWrite (RELAY, LOW);
    digitalWrite (GREENLED, LOW);
     
  }

  else   {
    Serial.println(" Access denied");
    digitalWrite (REDLED, HIGH);
    delay(3000);
    digitalWrite (REDLED, LOW);
  }

}

Moderator edit:
</mark> <mark>[code]</mark> <mark>

</mark> <mark>[/code]</mark> <mark>
tags added.

2

The line

 if (content.substring(1) == "85 A5 B0 56") //change here the UID of the card/cards that you want to give access

Should look for all the cards you want to have access to your system. If this is just a few use a compound if statement here with the OR function ||

 if (content.substring(1) == "85 A5 B0 56" || content.substring(1) == "23 E5 B0 28" );