In the Mog Station, under Square Enix Account Settings, click "List Subscriptions". It will take you to a page looking like the old account management site. Once there, look for the part which says "Billing Cycle". Next to that will be the amount of days your subscription is listed for. And next to THAT will be an UPDATE button. I BELIEVE, though I may be wrong, if you HIT that update button, you'll be able to select a longer/shorter option.

^_^ Let me know if that helps!