The terms and conditions say you can cancel or change any booking up to 15 minutes before your ticket departure, online or at one of the Flixbus travel agencies.
Having said that, I must clarify the cancellation policy is quite strict in terms of refunds.
You will get a cancellation voucher in the amount of the price ticket minus the cost of cancellation process. Those fees increase as it gets closer to the departure date. It is only free if you cancel your trip up to 30 days before your departure date.