To change your subscription type, go to the Package section from the menu. Here you can end your current subscription or switch to something else. If you choose to upgrade the package, the changes will take place immediately and you will receive a refund for the remaining days of your previous package. If you choose to switch to a smaller package, the switch will take place at the next payment.
If you pay through iTunes, the changes must be made through them. You must cancel your package before you can start a new one. Read more about how to cancel your subscription via iTunes here.