In the Payments tab, find the client whose invoice you'd like to cancel and click 'View.'
Towards the top right, just under the heading, you'll find the 'Cancel Invoice' button, right next to 'Mark As Paid'.
Click 'Cancel Invoice' and confirm this on the pop-up
That's it! You'll get a success message to show that this is complete and any logged sessions on that invoice will now appear as unbilled.