There are no contracts, so you are free to cancel at any time via the billing portal here on your account page. When you cancel your Infinite Uploads plan, the service will continue as normal until the end of your billing period. You can reactivate your plan at any time before the end of the period.
Important: You should use the "Disconnect" tool in the Infinite Uploads settings on each site to download any files that are only in the cloud to your local storage before your cancellation goes into effect to prevent any disruption for your sites and to protect your data.
At the end of your billing period the cancellation will go into effect. All new uploads will be blocked, your CDN will be disabled, and files scheduled for deletion.