If you need to extend your access to a Study-Pack for any reason (e.g. you moved your exam date), you can simply go to the Study-Pack Store and "add time" via an additional purchase.
On Memorang, subscriptions are automatically stacked so if you purchase additional plans, the time is additive. For example, if you purchased a 12 month subscription and then a 6 month subscription the next day, your total access time would be 18 months.