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 'My Pro Plan'.
Once the account is on the free plan, you can visit www.spreaker.com/plans 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 'My Pro Plan'.
You can then immediately activate again your plan selecting the yearly payment on https://www.spreaker.com/plans.