You can change your billing between monthly and annual terms.
From the My Account page,
1. Choose Manage Subscription
2. Choose Change Subscription
3. Choose length, then Sign Up Now
Switching your billing will not charge you anything until your next billing date, so you can easily switch back if you change your mind.
If you are currently signed up for monthly billing, switching to annual billing will charge you the annual subscription rate at the end of your current billing month.
If you are currently signed up for annual billing, switching to monthly billing will not start charging the monthly rate until the end of your current billing year.