how do i update firstName in

Question

i dont know how to add an update firstName function with my code. i am fairly new.

public class Contact {
private final String contactID;
private String firstName;
private String lastName;
private String Number;
private String Address;
private static AtomicLong idGenerator = new AtomicLong();
//CONSTRUCTOR
/*
 * The constructor takes first name, last name, phone number, and address as parameters.
 * The first thing it does is generates a new ID for the contactID field.
 *
 * First name and last name are checked for null condition or blank fields. If either of
 * those conditions exist, fill in the field with the phrase "NULL" so that something exists
 * to protect data integrity while making it clear it is a placeholder.
 * In both cases, if the first or last name is greater than 10 characters, truncate it
 * so that only the first 10 characters are used.
 *
 * For the number field, if the phone number is not exactly 10 characters then fill it with
 * the placeholder '5555555555'.
 *
 * Address is like first and last names. If it is blank or null, set it to "NULL".
 * If it is more than 30 characters, truncate to the first 30 characters.
 */
public Contact(String firstName, String lastName, String number, String address)  {

    //CONTACTID
    //Contact ID is generated when the constructor is called. It is set as a final variable and has
    //no other getter or setter so there should be no way to change it.
    //The idGenerator is static to prevent duplicates across all contacts.
    this.contactID = String.valueOf(idGenerator.getAndIncrement());

    //FIRSTNAME

    if (firstName == null || firstName.isBlank()) {

        this.firstName = "NULL";

        //If first name is longer than 10 characters, just grab the first 10 characters
    } else if(firstName.length() > 10) {
        this.firstName = firstName.substring(0, 10);
    } else {
        this.firstName = firstName;
    }

    //LASTNAME
    if (lastName == null || lastName.isBlank()) {
        this.lastName = "NULL";
    } else if(lastName.length() > 10) {
        this.lastName = lastName.substring(0,10);
    } else {
        this.lastName = lastName;
    }
    //NUMBER
    if (number == null || number.isBlank() || number.length() != 10) {
        this.Number = "5555555555";
    } else {
        this.Number = number;
    }
    //ADDRESS
    if (address == null || address.isBlank()) {
        this.Address = "NULL";
    } else if(address.length() > 30) {
        this.Address = address.substring(0,30);
    } else {
        this.Address = address;
    }
}

//GETTERS
public String getContactID() {
    return contactID;
}

public String getFirstName() {
    return firstName;
}

public String getLastName() {
    return lastName;
}
public String getNumber() {
    return Number;
}
public String getAddress() {
    return Address;
}

}

package ContactService;

import java.util.ArrayList;

public class ContactService {
//Start with an ArrayList of contacts to hold the list of contacts
ArrayList<Contact> contactList = new ArrayList<Contact>();

//Display the full list of contacts to the console for error checking.
public void displayContactList() {
     for(int counter = 0; counter < contactList.size(); counter++) {
        System.out.println("\t Contact ID: " + contactList.get(counter).getContactID());
        System.out.println("\t First Name: " + contactList.get(counter).getFirstName());
        System.out.println("\t Last Name: " + contactList.get(counter).getLastName());
        System.out.println("\t Phone Number: " + contactList.get(counter).getNumber());
        System.out.println("\t Address: " + contactList.get(counter).getAddress() + "\n");
    }
}

//Adds a new contact using the Contact constructor, then assign the new contact to the list.
public void addContact(String firstName, String lastName, String number, String address) {
    // Create the new contact
    Contact contact = new Contact(firstName, lastName, number, address);
    contactList.add(contact);
}
public void removeContact (String firstName, String lastName, String number, String address) {
    Contact contact = new Contact (firstName, lastName, number, address);
    contactList.remove(contact);
}
public void updateFirstName {
    
}

}

how do i get the updateFirstName function to work. i need to be able to update the first name within a contact, but i dont know if i need to create a new contact like what was done in the add and remove contact functions. ive tried contact.setfirstname = new firstName but that did not work and i am at a loss on what to do.