Should you select the monthly plan initially and decide you would like to save 30% by upgrading to the annual subscription, click on Account on the Navigation Bar.
Once there, click on Update Subscription.
This text box will appear and you will click on your current subscription, Monthly RUN.
You'll then see this screen showing both Annual Run and Monthly RUN:
Click on the Rank Up Now Yearly Subscription tab, and then on Update Subscription. Your subscription will then be updated.