How can I withdraw a manuscript that is placed under-review?
Just contact the editor in charge of your submission.  It may not be the main editor but an associate editor. Usually, when you login in the website for managing submissions, there is often a link to contact the editor. If not, you may also know who is the editor in charge of your submission by reading the e-mail that was sent to you as confirmation when you submitted the paper. Otherwise, you may look for an e-mail on the website to contact the editor. Those are general guidelines. It may vary depending on the journal. But don't worry. It is not a problem to remove a journal paper. This happen quite often.