Most coach fares in the region are final. You buy a seat on a specific departure and that is the seat you have. Operators price this way because a coach that leaves with an empty seat has lost that fare for good, and the low prices you see on the results depend on it. It is fair, and it is also, occasionally, a problem, because plans change. Flexible Ticket exists for exactly that gap.
Flexible Ticket is an optional add on you can choose at checkout. It costs a small percentage of the fare per passenger, with a modest minimum, and it is shown clearly before you confirm. What it buys is the right to cancel your booking for a refund or to move it to another departure, within a window before travel that the operator sets. That last part matters. Different companies allow different windows, and the results tell you what applies to the coach you are looking at before you pay rather than after.
It is not a promise that you can change your mind on the platform. Windows close some time before departure, typically a couple of days for a cancellation and a little longer for a change of date, because the operator needs a real chance to sell the seat again. It does not cover the small add on fee itself, and it does not turn a discounted round trip into two separately refundable singles. If you booked without it, the standard fare rules of the operator apply, which usually means the ticket is not refundable. We are deliberately plain about this, because an add on that looks like insurance and behaves like small print helps nobody.
If you are booking weeks ahead for a trip that depends on other things, a flight, a job interview, a wedding that might move, the few percent buys real peace of mind. If you are booking for tomorrow and you know you are going, it is money you do not need to spend. Travellers with fixed plans skip it, travellers with moving parts take it, and both are right.
Everything happens from your booking page, which you reach from the confirmation email or from your account. If the window is still open you will see the option to cancel or to pick a different departure, and the system tells you the deadline in the departure city's local time. Refunds go back to the card you paid with. If you move the date to a departure that costs more, you pay the difference, and if it costs less the difference comes back to you.
The one thing we would add is a habit rather than a feature. Decide about Flexible Ticket when you book, not when you need it, because by then the moment has passed. It is a small tick box on the checkout page, and for the right trip it is the best value item on the whole receipt.