If you have an active subscription, you can change plans at any time. For information in regards to when you'll be charged for changing plans, we recommend reviewing this article: I want to change subscription plans. When will I be charged?
1) Navigate to your My Account page
2) Select Subscriptions/My Subscription from the left-hand side menu.
3) If necessary, select your active subscription. Then, in the Subscription totals section, hit the Change Billing Period button.
4) Finish checkout!
Once clicked, you will be taken to the checkout screen with the other plan automatically selected (i.e. if you're on Yearly, it will select Monthly - and vice versa).
Fill in your payment details and finish the checkout process.
5) Once the process above is completed, you should be able to see your new plan shown in the Product section under Subscription Totals (as seen previously).