Update data from firebase for NodeJS

I have created a table in firebase. This is how my code looks like

function writeUsersData(id_user, id_legal_user, balance, add_balance, date, status) {
  firebase.database().ref('users/' + id_user).set({
    id_user: id_user,
    id_legal_user: id_legal_user,
    balance: balance,
    add_balance: add_balance,
    date : date,
    status: status
  });
}

//writeUsersData('1', '1', '99.99', '0.99', '2014-11-12T11:01:00-05:00', 'active');

And I want change add_balance, how do I do this? If I create new method from query for two rows (id and add_balance) for 'set' this rewrite(