balance = JFrame.getInstance().getBalance();

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.text.DecimalFormat;
import java.io.*;
import java.util.*;
 
public class atmEngine extends JFrame 
{
 
    private double balance; //Here is where balance is set in the class
    private TitlePanel title;
    private FunctionPanel function;
    private AccountBalancePanel info;
    private NumberPanel numPad;
    private FastCashExitPanel fastCashExit;
 
    public atmEngine() throws IOException
    {
        //Window Title
        setTitle("Jon's ATM Interface");
 
        //Using close button
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 
        //creating layout manager
        setLayout(new BorderLayout());
 
        //setting size
        //setSize(700, 10800);
 
        //Creating Panels
        title = new TitlePanel();
        function = new FunctionPanel();
        info = new AccountBalancePanel();
        numPad = new NumberPanel();
        fastCashExit = new FastCashExitPanel();
 
        add(title, BorderLayout.NORTH);
        add(function, BorderLayout.WEST);
        add(info, BorderLayout.SOUTH);
        add(numPad, BorderLayout.CENTER);
        add(fastCashExit, BorderLayout.EAST);
 
        pack();
        setVisible(true);
 
    }
    public void setBalance(int accountNumber) throws IOException
    {
        String accountFile="accounts/accounts.csv";
 
        int accountTranslation=0;
        boolean accountVerified=false;
 
        FileReader freader = new FileReader(accountFile);
        BufferedReader outputFile = new BufferedReader(freader);
 
        String str="404";
 
        while((str!=null)&&(accountVerified!=true))
        {
            str = outputFile.readLine();
            if ((str!=null) || (accountTranslation==accountNumber))
            {
                String[] tokens = str.split(",");
                accountTranslation = Integer.parseInt(tokens[0]);
 
                if(accountTranslation==accountNumber)
                {
                    balance=Double.parseDouble(tokens[1]);//here is where to find balance
                    accountVerified=true;
                }
                else
                {
                    accountVerified=false;
                }
            }
        }
        if(accountVerified==false)
            JOptionPane.showMessageDialog(null, "Account does not exist");
 
    }
//this method returns balance
    public double getBalance()
    {
        return balance;
    }
}

import javax.swing.*;
import java.awt.*;
 
public class AccountBalancePanel extends JPanel
{
    //private double b=atmEngine.getBalance();
    public boolean withdrawDeposit = true;
 
    private JLabel balance;
 
    public AccountBalancePanel()
    {
 
        balance = new JLabel("Account Balance: $0.00");
 
        setBorder(BorderFactory.createTitledBorder("Current Information"));
 
        add(balance);
    }
//below is the constructor that accepts the balance
    public AccountBalancePanel(double accBalance)
    {
 
        balance = new JLabel("Account Balance: $"+accBalance);
        System.out.println(accBalance);
        setBorder(BorderFactory.createTitledBorder("Current Information"));
 
        add(balance);
    }
}

If you want class ATM to be able to update class BankGUI, then the constructor for ATM would look like this: 
 
public ATM(BankGUI theGUI, ....){
atmsBankGUI = theGUI; //ATM has to have a variable called atmsBankGUI
}
 
And lets say ATM lets you withdraw money. 
 
public void withdraw(){
int amount = dollarAmount; //get the dollar amount the user wants to withdraw
atmsBankGUI.withDrawCash(amount);
}
 
 
//Now, this method would be in the BankGUI class
 
withDrawCash(){
//Do whatever you have to, to make the GUI display the withdrawal
//For example, you could use JLabel.setValue to set it to the new amount of money the user has.
}