...If you have the original confirmation email from Genius at Apple store, you
can reply through that - to change or cancel - the appointment reservation...
{The email would have been sent to your account email as Apple ID, used to login here.}
You may also use the http://www.apple.com/retail/storelist/ and lookup your Store
where Genius appointment was set, select Store and see if there isn't a link to page
that you can select to change or cancel Genius bar appointment. You may also see
(while logged in via AppleID) your existing appointments more readily.
Not sure which official Apple store Genius locations have and use Chat as part of the
connectivity to help customers resolve scheduling, service, or support issues. The
suggestion some likely do, is noted here: https://www.apple.com/retail/geniusbar/
Some users said that if nothing else, they accidentally made a second appointment;
then when they sought to edit, the ability to change or remove the first one appeared.
One who spoke to an Apple store employee (a year ago) said she was told to just
make another appointment; as someone else would be moved ahead in time to
the vacated or unfilled time slot when the initial appointee did not appear.
Several dated bits I found online note changed procedures at the Genius bar in
Apple stores; but this seems to be current. However if using an iPhone or iPad,
your ability to access a Genius bar is likely best; scheduling & settings improved.
That said: your actual experience may vary depending on location, region, & country.
Took me awhile reading several sources including Apple pages, other ASC discussion
posts; so try the first items above to contact for reschedule or cancellation.
Good luck in any event...! 🙂