You can upgrade, downgrade or cancel your plan any time in Billing settings. On upgrade, your payment will be prorated so you will only pay for the difference between the old and the new price, taking into account how much of the billing cycle has already passed. On downgrades, there is no proration, except for the unused platform usage credits that are rolled over to the new plan. If you decide to cancel, your subscription will last until the end of the current billing cycle, and it won't be renewed after that. Note that you might still be charged for excess platform usage.