Cancellation

https://www.apple.com/au/retail/geniusbar/

select your store.

The next screen has change or cancell a reservation.

W are not Apple employees, rather end users like you and cannot do anything about appointments. You'll need to call the store or start with this link:

Apple Store - Find a Store - Apple

Hopefully that will lead to a link for your country. Once you identify the store you should be able to find the cancel function.

We are not Apple. We are users like you. If you want to contact Apple call the store and let them know.