If you'd like to change your Growth plan from monthly billing to annual billing (annual billing gets you two months free!), you can do that anytime from the billing page.
(Note that the Operations plan is only available on monthly billing.)
Navigate to the billing page by clicking Settings > Billing in the left hand sidebar.
Then click "Change plan" in the top right.
You can then choose to upgrade or downgrade your current Growth plan on monthly billing, or click "See Annual Pricing" to choose an annual plan.
Changing your plan from annual to monthly is a similar process.