Every booking in your account shows the operator's refund rule and what cancelling today would give back. Pressing Cancel this booking only opens a form, so you can always change your mind.
In My bookings, find the trip under Coming up and press Open booking.
Scroll down to the Cancelling card. It lists how much comes back depending on how early you cancel.
The line under the rule, such as You get all of it back, says what cancelling today would return.
Press Cancel this booking. A short form opens; nothing is cancelled yet. You can add a reason in Why are you cancelling?
Press Yes, cancel it to cancel; the refund goes back the way you paid. Press Keep my booking to close the form and change nothing.
Careful. Cancelling cannot be undone, and the closer the trip, the less comes back. Read the line in Cancelling first.
Tip. If a booking is past its cancellation window, the card shows a link to contact support instead of the button.