Ted here from Shopify, thanks for posting your query!
Sure thing, so you should be able to switch back to the monthly plan by selecting Settings > Plans and Permissions > Compare Plans in your admin. After that, you'll notice the monthly plan options will appear as shown in this screenshot. If you would like to view the annual price plans again you can do so by selecting "change your billing period" in the blue banner as shown in the last screenshot.
How are you finding Shopify so far? If there is anything else I can help you with, please let me know, I'm happy to help!