Subscriptions are for the period selected when they are taken out and, once purchased, cannot be extended. Once your subscription to ONtrack has expired, simply sign in again and try to start a question session. You will be redirected automatically to the subscription purchase page.
Provided that you use the same email address and password for the new subscription as you did before, your previous sessions will still be available for you to review.
Please note that the converse also applies. Currently, the only way to resubscribe but not continue from where you left off, i.e. to start again with a clean slate, is to create a new ONtrack user account identified by a different email address. (You will thereby retain the opportunity to look back at your previous sessions if you use your original login.)