You can see if this works but it may not change you to monthly right away if you have already signed up for a year.
The standard way would be to cancel the subscription entirely and then ask for a refund. However, you wish to change it to a monthly rate and presumably just pay for the first month already used and get the other 11 months refunded while still maintaining a subscription. You would have to ask Apple if that was possible. Otherwise you might just have to cancel the subscription entirely, ask for a refund, then resubscribe.
contact Apple online using this page ➔ Choose your country or region - Official Apple Support
If you have to do it as a two step process, review the information in this support article by clicking on the link to learn how to cancel a subscription from Apple. ➔ If you want to cancel a subscription from Apple - Apple Support
• "If you signed up for a free or discounted trial subscription and you don't want to renew it, cancel it at least 24 hours before the trial ends."
• An existing subscription will normally continue until the end of its term and may then renew automatically. Stopping automatic renewal is effectively canceling it. "If you don’t see Cancel Subscription, then the subscription is already canceled and won't renew." The "Expires" date means the current subscription will just run until then.
• Read the article to see what to do about subscriptions that do not appear in your list. If you still can't find a subscription, contact Apple Support using the link at the end of the support article.
• Canceling a subscription does not automatically start a refund.
Finally, follow the directions in this support article about how to request a refund for apps or content that you bought from Apple ➔ Request a refund for apps or content that you bought from Apple - Apple Support