You can cancel a request through your account on the platform by clicking the "Cancel" button:

Then choose a cancellation reason from the dropdown and, if you'd like, explain why you want to cancel. Click "Cancel request" to confirm:

If the lesson can no longer be canceled on your own, you’ll get the following message:

contact the tutor so they can do it.
The tutor's details appear in the "My tutors" tab in your account on the platform:

Next to the tutor's name, you'll see a phone-shaped button.
Clicking it reveals the tutor's contact details, which you can use to get in touch and ask to cancel the request/lesson: