At the bottom, click on "Manage Carousel Studio plan" and it should take you to the Settings > Billing.
There you can cancel your subscription and find invoice details as well
FAQ
What email should I use to cancel my subscription?
Use the same email you used to pay for the subscription. You can see this in Settings > General > Billing email. This would be the email address where you got the upgrade email in. This is NOT not your Canva account email.