I'd like to change my plan from 3 months to 12 months. How can I change this? I cannot find this option. In addition, I'd like to know how I can cancel my plan if I need to.
You should be able to use the Power Up Sync to choose the billing plan desired (3 or 12 months).
You can find more information on the F-Secure Key app page about billing.
"Auto-renewal may be turned off by going to the user's Account Settings after purchase."