You need to first cancel (or pause) your current subscription. If you're unsure how long you've had your subscription, check in the WOW app under "My Subscriptions."
If you already have an active WOW subscription, use the "Cancel WOW" button to deactivate it. Otherwise, the regular WOW subscription will extend indefinitely.
Once your current WOW subscription is deactivated, you can start anew with your WOW TV gift card. The subscription ends automatically when the duration expires.