After the information you enter to buy Rotterdam bus tickets, the system will list all the tickets. You can get the cheapest Rotterdam bus ticket by sorting this list by price.
You can buy a new ticket with your ticket amount by canceling the Rotterdam bus ticket you have bought until the last 1 hour before. This is how you change it. If the new ticket you buy is cheaper than the ticket you changed, the difference can be refunded or deducted from your next ticket by remaining open.