ANDROID - FIREBASE UPDATE OPERATION

I want to update my record in my firebase database table. I will update order cost as 10 which has primary key as: "20170807192734".

Here is the query I should write to update:

final FirebaseDatabase firebaseDatabase = FirebaseDatabase.getInstance();
DatabaseReference dbRef = firebaseDatabase.getReference("orders");
String primaryKey = "20170807192734";
dbRef.child(primaryKey).child("cost").setValue("10");

Firebase veritabanimdaki orders tablosundaki "20170807192734" primary key e sahip olan kayitin costunu 10 olarak guncellemek istersem yazmam gereken kodlar yukaridaki gibi olmalidir.