The fare decides, and it decides when you buy
Tickets are nonrefundable and nontransferable unless the fare says otherwise. Every result and every checkout says which yours is, on two lines — changes and cancellation — each marked free, for a fee, or not allowed.
Those two are separate. A fare can let you cancel and still return no money: cancelling gives up the seat, a refund gives back the fare, and only a refundable fare does both.
On a connection, the tightest flight decides
A journey is only as flexible as its strictest leg. A fare that forbids changes on the second flight forbids them for the whole journey, whatever the first flight allowed.
Cancelling a booking
Open My bookings and cancel it from the card for that trip. You will be asked to confirm first. It then moves to cancelled bookings and keeps a page of its own, so you can still send it to someone — but the flights are no longer held and the reference will not check in.
Changing dates
Not something this site does. Where the fare allows changes, ask the airline: it sets the fee and charges any difference in fare. Where it does not, the only way round is to cancel, if the fare lets you, and book again.
Getting money back
Whatever the fare allows is settled by the airline, so quote your booking reference to it. The booking’s own page holds the contact details given at checkout, and the airline’s page here holds its phone number.