Add/Increment Value - Firebase Database w/ Swift

Question

I'm new to Xcode and I want to know how to add/increment value instead of set value. Appreciate your help in advance.

import UIKit
import FirebaseDatabase

class Feedback1Controller: UIViewController {

    var ref: DatabaseReference?

    @IBAction func Button3(_ sender: Any){
    ref = Database.database().reference()
    ref?.child("FavBooth").child("Booth A").setValue(1)
    }

Image of my Firebase Database