Monthly to Annual
You can switch your subscription from monthly to annual by canceling your current subscription and then re-subscribing for an annual subscription. To cancel your current subscription, you will need to cancel your auto-renewal through your device.
You can access your device's subscription settings through the iPhone Subscriptions page. Once there, you should see the Aurora subscription listed with an option to cancel. If you see Aurora listed, but it only shows an expiration date and no option to cancel, then the subscription has already been cancelled.
Annual to Monthly
We, unfortunately, do not have any way to reduce your annual subscription to a monthly subscription. You can cancel the subscription on your device following the instructions above, but since you've already paid, even if you cancel you'll still have access to the premium content until the annual subscription ends next year.