To cancel your subscription, go to Settings > Billing > Manage Subscription. At the bottom, click Cancel My Subscription.
If Billing > Manage Subscription looks like the image below, click Cancel My Subscription to change your plan.
Follow the prompts and complete the survey, and make sure to choose Complete Cancellation. You will get a confirmation of the cancellation in your email inbox as well to the owner address on file.
Once you have canceled your subscription, any remaining subscription time in your billing cycle will go onto your account as trial time. You will maintain access to that calendar until that time runs out.
To completely delete your calendar or account, check Closing Your Account.