In order to change your current monthly PREMIUM subscription/billing to a yearly one, you'll have to manually buy a new yearly subscription or vice versa.*
Please note that the payment for the new monthly/yearly subscription will be processed immediately but your current subscription will still run until the date of expiration/renewal. It is going to be replaced subsequently afterwards so that you won't lose any paid term.
A cancellation of your current subscription is not necessary in order to switch from monthly to yearly subscription or from yearly to monthly subscription.
* This only applies to subscriptions which have been booked directly through Zattoo's own web shop. If you've booked a subscription through one of our partners, then please contact the official support of our partners regarding questions about contract or payment related issues.