To cancel a reservation you've made as a guest, the most efficient way to do it is through the Confirmation email you've received for that specific reservation.
Find and open the Confirmation email.
Scroll down until you find the grey View or Modify button.
Click or tap on it.
A new window will open. Click or tap on the white Cancel button.
Next window will ask you to provide a reason of cancellation.
You can select one of the available ones from the drop-down menu
or just select Other.
Click or tap the Confirm button.
Close the window.
Check your inbox, you will receive an email that confirms a cancelled reservation.