Calling a function from class without object

#include <iostream>
using namespace std;
class BankAccount{

    string f_name,l_name,acc_type;
    long balance,acc_num;
    public:
    //cunstructor to set values automatically
    BankAccount(string fname, string lname, string acctype, long bal){
            f_name=fname;
            acc_type=acctype;
            l_name=lname;
            balance=bal;
            if(balance<50000)
            {
                acc_type="Basic";
            }
    }
    //displays menu 
   void displayMenu()
    {
        cout<<"Select an option\n\n1.Deposit/withdraw\n2.Change Account type\n3\n4.Change Name\n5.Display Details \n ";
    }
    //let user change name 
    string changename()
    {
        cout<<"Your current name is "<<f_name<<" "<<l_name<<" Enter your details below to change it\n";
        cout<<"Enter your first name \n";
        cin>>f_name;
        cout<<"Enter your second name \n";
        cin>>l_name;
    }
    //returns first name;
   string getf_name()
    {
        return f_name;
    }
        //returns second name;

   string getl_name()
    {
        return l_name;
    }
    //returns account number
    long getAccountNumber()
    {
        return acc_num;
    }
    //let user change account type
    string changeAccountType()
    {
        char ch;
        if(balance<50000)
        {
            cout<<"Account type can't be changed because balance is less than 50,000. You need to maintain atleast 50,000 or more in your account if you want to switch to current account\n";
        }
        else
        cout<<"Your Account type "<<" "<<acc_type<<" Do you wish to change it to Basic? \n";
    }

        //displays all account details
//let user deposit and withdraw 

int depositWithdraw()
{
int n;
int amount;
cout<<"Do you want to deposit or withdraw\n 1. Deposit \n 2.Withdraw"<<endl;
cin>>n;
if(n==1)
{
cout<<"Enter amount to deposit\n";
cin>>amount;
balance+=amount;
}
else if(n==2)
{
cout<<"Enter Amount to withdraw\n";
cin>>amount;
if(balance<amount)
{
cout<<"You don't have enough balance\n";
}
else if(n==2)
{
    balance=balance-amount;
    if (balance<50000)
    {
        acc_type="Basic";
    }

}

}
}


void displayDetails()
{
    cout<<"Name: "<<f_name<<l_name<<endl;
    cout<<"Account type: "<<acc_type<<endl;
    cout<<"Account Balance "<<balance;
}
};

int main()
{
            BankAccount customer1("Hamza","Jameel","current",25000);
            BankAccount customer2("Bilal","Aziz","current",25000);
            BankAccount customer3("Raja","Ali","current",60000);
            BankAccount customer4("Abdullah","Shafique","basic",75000);
            
            displayMenu();

            customer1.changeAccountType();
}