Switching from one plan to another is a really simple thing to do! This can be done from the Billing section of your account. 

On the Project Dashboard, click your profile image in the bottom left-hand corner. From the menu, select Billing & Invoices. 

Select Open Billing Dashboard.

Here, you can choose which plan you'd like to switch to. Any previous payments you've made will be taken into account and the next payment will be calculated on a pro-rata basis. 
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👆 Note: Opting for yearly billing means that you'll save two months worth of payments over the course of a year 💸