Sure you are free to change but you will not get any fund back... so it's "not refundable".

So you can pay $145 + GST to change it. This is how you do it. If you are sure you need a new date/flight, then call and change it on the phone, they will charge you the fare difference (there are different fares within same group such as V or L) PLUS this $145.

If you don't have any new plan and just want to cancel the trip. Then call to cancel the reservation, your ticket is usually good for 1 year. Any time during this year you will apply the value of this ticket - $145 to buy a new ticket.

Any travel agent can accept your unused ticket for credit but it might take them a few days to sort it out first. Some will just deny accepting because of the paperwork troubles involved.