You can change your billing period from annual to monthly on the Go Pro page. Just switch the page to monthly plans and click on "Extend".
Then you should pay for a monthly subscription. With that being done, your plan will be extended by 30 days. When the current subscription is over, you will be charged monthly.
If your subscription was purchased in our mobile app via Google Play or App Store, the billing period cannot be changed. If you want to pay monthly, you should cancel the auto-renewal of your current annual subscription and wait for it to expire. After that you'll be able to purchase any monthly subscription plan.