Need to know your agenda for the day? Ask Siri, “How does [today/date] look?” If you have a place tagged with the event, you can ask Siri, “Where is my next meeting?”
Have an emergency that's thrown off your plans? Use keywords such as “Cancel [meeting name]” or “Move [meeting name] to [date/time].” If Siri finds a conflict with a previously scheduled event, she will ask you if you want to schedule anyway.
To cancel the previous meeting, say, “Cancel [meeting name].” If you want to do neither, simply tell Siri a different date and time for the meeting you are attempting to schedule.