If you have an iOS monthly subscription and wish to upgrade to the yearly plan, you are able to do so by going to "Settings" >> "Profile" >> "Edit Profile":
This will automatically open a new Window on your default browser that will allow you to edit your subscription:
You will see a list of the plans you are able to upgrade to. The upgrade will be effective immediately after purchase.
Enter your payment details, name and e-mail and click "Upgrade Subscription":
(!) Please note that you will have to cancel your ongoing iTunes subscription after the upgrade. Click here for more details.