You can upgrade your plan at any time, and your limits will increase immediately. Any additional features included with your new plan will be available as soon as you upgrade. When upgrading your subscription, the unused time on your previous plan will be applied to your new plan, and you will pay a prorated subscription rate.
When you downgrade to a plan with lower limits, these will be effective immediately. The unused time on your plan will be added to your account as a balance and will be used for your next invoice.
If you would like to cancel your subscription and move to the Starter (free) plan, follow this guide. When you cancel a paid plan, you will still stay on your existing plan until the next billing date, and then you will be downgraded to the Starter plan.