Not sure if you mean for electricity or gas, but for electricity you can easily update to monthly by the app, or in your logged in AGL account online.
With the app, you click the settings icon on the bottom menu, then settings, then you'll see monthly billing down the list a bit.
Click that an change it over.
On the website login to your account, click billing menu at top, then payment options - all payment options, click monthly billing, edit, choose your service (you must be able to do gas too), then click the acknowledgment box and click change to monthly billing.
