Scroll down to Course Subscription. And select the course you wish to extend (i.e. UKCAT).
In the next page you can select the length of the extension needed.
Don’t worry about your account expiring: you’ll also be given these renewal options on your next visit, and your data is not lost.
Extensions do not reset your account, so you will not be able to re-take the single-use mock exams, nor zero your practice data.