You pay nothing now for cancelling the flight.

You will be able to use the credit associated with the booking until the original form of payment expires, which will be in December. After this time you will lose all of the value.

To rebook, you use the booking reference number associated with this ticket. You can search for new flights and it will apply the value of your credit towards the new ticket. In addition, you will be charged a $150 fee at the time you perform this rebooking.

But you will have to book flights that are within 12 months of your original voucher being issued - so within the next few weeks from what you describe above.