Yes you can; here are the rules from Westjet's website:
Change your reservation within 24 hours of booking
(up to two hours prior to departure) Changes made through our website will have a $50 CAD/USD fee (plus tax, per person). There is no charge if you call our Sales Super Centre at 1-800-581-9499. Any change will be subject to any difference in fare.
Change your reservation after 24 hours of booking
(up to two hours prior to departure)
A $50 CAD/USD change fee (plus tax, per person) will be charged, based on the type of fare being changed. Any change will be subject to any difference in fare.
Cancel your reservation within 24 hours of booking
Please note: if you book a flight for travel today (also called same-day travel), you must cancel more than two hours from your flights scheduled time of departure.)
A refund will be returned to the original form of payment by calling our Sales Super Centre at 1-800-581-9499. If more than one method of payment was used for web bookings, a travel bank credit will be issued.
Cancel your reservation after 24 hours of booking
(up to two hours prior to departure) A $50 CAD/USD fee (plus tax, per person) may be charged, depending on the type of fare being cancelled. The remaining funds will become a travel bank credit.