It depends on what fare category your ticket is under. Check your fare rules but usually cancellations can be made up to 45 minutes prior to departure of the flight. If you cancel, you will get a refund for the the value of unused ticket which can be applied to the value of a new ticket with a few stipulations ...
From the Air Canada website:
... subject to a $75 CAD change fee per direction, per passenger, plus applicable taxes and any additional fare difference, subject to availability and advance purchase requirements. The new outbound travel date must commence within a one year period from the original date of ticket issuance. If the fare for the new journey is lower, any residual amount will be forfeited.