To change the length of your subscription, sign into your account page here. When you click on 'My Subscriptions' you will see the option to 'change duration' at the bottom of the page.
When you click this, it will take you to the check out process. All you have to do is choose your new subscription length and complete the check out process. This will cancel and replace your previous subscription and change your billing date to the date you are ordering.
Please be aware that gift subscription durations are unable to be changed. If you would like to extend a gift subscription, you will have to place a new order for a subscription.