Monthly to Annual
You can switch your subscription from monthly to annual by cancelling 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.
On your iPhone
You can access your device's subscription settings through the Suggestic app 'Subscription' section. Tap the option at the bottom to be redirected to your iPhone's iTunes settings. Once there, you should see the Suggestic subscription listed with an option to cancel. If you see Suggestic listed, but it only shows an expiration date and no option to cancel, then the subscription has already been canceled.
Annual to Monthly
Unfortunately we don't 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.