You will be able to do it directly within your apt scheduled in the "Review" section or from the "TaxExpert" button your appointment will be listed and you will have the option to cancel it.
Unfortunately, there is no option to cancel an appointment I made. I received an email saying there would be a banner with my appointment time to click on to make changes, but no such banner exists! Escalated the issue with TurboTax tech support on Feb.16,2021. Frustrating!!