To cancel individual IP addresses, go to the billing page. Then find your order that you wish to downgrade. Click on the small arrow next to the "View" button and click on "Modify." There you can select IP addresses that you wish to cancel.
Please keep in mind that this page is only for canceling individual IP addresses. If you want to cancel all of your IP addresses in your order, you should cancel the whole order instead.
If you don't know which order your IP address belongs to, paste your IP address into the search box in the sidebar and press "Enter". There you can see all information you need to know about the order and IP address.