Once you have signed in to your Library account, on the main page you should see a series of tiles, including one titled "Requests". Any books you have requested should be listed there.
Click on the book request that you would like to cancel. This should take you to the book's listing under the "Requests" tab, and you should now see a "Cancel" link next to the listing.
Following this link you should then be asked to confirm the cancellation, and then the request will be cancelled. You will receive an email confirmation.