The front screen of italki will show the upcoming lesson. If you click on where it says 'upcoming lesson', then a new screen will appear showing the details of the lesson. There are options on the right hand side and one of these says, 'cancel'. I hope that helps.

Oh, ok. Mine shows an option for Change date/time. Maybe it depends on how soon the lesson is?

Actually, my lessons are part of a package, so maybe that's the difference. Perhaps single lessons are just cancelled and then rebooked.