The author is not allowed to withdraw submitted or published manuscripts (unless there are compelling reasons), because the withdrawal is a waste of valuable resources that editors and reviewers spent a great deal of time processing submitted manuscripts and works invested by the publisher.
If the author still requests the withdrawal of his manuscript, the author will be required to submit an "Article withdrawal form" signed by all the authors (or) authors of the corresponding manuscript stating the reason for the withdrawal of the manuscript. Authors should not assume that their manuscript has been withdrawn until they receive appropriate notice for this effect from the editorial office. Please contact firstname.lastname@example.org com for an "Article withdrawal form".
However, it is unethical to withdraw a submitted or published manuscript from one journal if accepted by another journal.