You can only make this change when renewing your policy. Then you'll be able to change your payment frequency from monthly instalments to one annual payment, or the other way around.
To change the frequency while your policy is renewing simply login and select the Renew My Policy box for the policy you want to update. (see image below).
This takes you to the Renew Policy page. Under the heading - Select your payment frequency you'll see the Pay Annually box and the Pay Monthly box. The one in green is the one that's active. In the example below the Pay Monthly option is active. Click on Pay Annually and it will turn green to show that you've changed to a yearly payment. If you need to make any other changes to your policy like adding optional extras, kilometres driven, car usage, financing or listed drivers, select Yes and Continue.
On the Confirm renewal changes page hit Confirm if you're happy with your new payment frequency, and other selections.
From the Bingle Secure Transaction page, select the credit or debit card type and enter the name on your card, your card number and the expiry date. After this, hit Pay now and your annual (or monthly premium) will be paid.