To change from the yearly to the monthly plan, you need to downgrade the account to cancel at the end of the billing cycle. You can find the cancellation button on your settings under Plan and billing info.

Once the account is on the free plan, you can visit our plans page and re-upgrade to the account using the monthly payment option.

If you want to switch from a monthly payment to a yearly plan you need first to cancel your current subscription. You can find the cancellation button on your settings under Plan and billing info, then wait for the billing cycle to end. You will not lose anything in the meantime and the plan will work regularly.
Then from the Plans page, after that date, you will be able to purchase the yearly plan.