I have signed up to the paid monthly subscription, however, I would like to find a way of reverting back to the free version. 

I have signed up to the paid monthly subscription, however, I would like to find a way of reverting back to the free version. 

You can easily downgrade to "Free" plan by following this guide: http://www.jotform.com/help/341-How-to-Downgrade-Subscription-back-to-Free-Account.

After clicking the downgrade button, it will schedule your current subscription plan to expire on the next billing date. That's when you will revert back to "Free" plan, from there you will not be billed/charged further anymore.

Do let us know if you need more information on this.