You would need to log into your HowDidiDo account and go to the Booking tab:
Then scroll down to the Upcoming bookings.
If you have a booking where you are the only player then you'll have the option to cancel the booking:
If you are in a booking that has more than one player you'll have the option to withdraw shown in the example below:
Or if you click on the booking you will be redirected to the tee sheet and then have the option to cancel the booking: