The Free plan allows for a single project and one account owner. In order to downgrade:
First, remove your projects: In order to cancel your subscription to the Free account, you must remove all but 1 Project on your account. This includes both Active and Archived Projects.
Next, remove your Teammates. If you are downgrading from a Company plan you will need to remove all teammates except for the account owner.
Finally, you can downgrade your account.
For your next project, you can easily upgrade back to a paid plan at any point.