Firebase CRUD, issues with update and delete

I'm new with firebase so I tried creating a simple CRUD function where I'm only able to do the saving of data. I will not be using the newPostKey method because there's only one user on this website. I would like to update the user_name and delete the entire data using the plate_no value but my code is not working. Here's my code:

    function save_user() {
        var user_name = document.getElementById('user_name').value;
        var plate_no = document.getElementById('plate_no').value;

        var data = {
            user_name: user_name
        }

        var updates = {};
        updates['/users/' + plate_no] = data;
        firebase.database().ref().update(updates);

        alert('The User is Created Successfully!');
        reload_page();
    }

    function update_user() {
        var user_name = document.getElementById('user_name').value;
        var plate_no = document.getElementById('plate_no').value;

        var data = {
            user_id: plate_no,
            user_name: user_name
        }

        var updates = {};
        updates['/users/' + plate_no] = data;
        firebase.database().ref().update(updates);

        alert('The User is Updated Successfully!');
        reload_page();
    }

    function delete_user() {
        var user_id = document.getElementById('plate_no').value;

        firebase.database().ref().child('/users/' + plate_no).remove();
        alert('The User is Deleted Successfully!');
        reload_page();
    }

    function reload_page() {
        window.location.reload();
    }