If you want to switch to a yearly subscription from a monthly subscription you will need to let your current subscription lapse. Your account details page will tell you how much time is left in your current subscription. Make sure that you do not have automatic recurring payments enabled. When your current subscription ends your account will be suspended.
Don't panic. When your subscription is suspended all of your recipe and meal plans are will still be in your account, but you will not be able to access them until you renew your subscription.
When you renew your subscription you will have the opportunity to choose between a yearly and monthly subscription.