Go to the Events tab. Under each event listing, you'll see a button for Edit Details. From here you can select one of these three options from the section. Save and continue to push the event change through.
If your event is being Ticketed by Eventfinda, you'll need to contact the ticketing team directly to flag your event as Cancelled or Postponed in case there are purchases that need to be managed. Contact the ticketing team at firstname.lastname@example.org
We don't delete cancelled events from the site as it's important people can see it's been cancelled rather than thinking they can't find it and it's still on.