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 withdrawal of his/her manuscript the author will be asked to submit an "Article withdrawal Form" signed by all authors (or) the corresponding author of the manuscript stating the reasons for manuscript withdrawal. Authors must not assume that their manuscript has been withdrawn until they have received appropriate notification to this effect from the editorial office. Please contact the Editor for the "Article withdrawal Form".
However, it is unethical to withdraw a submitted or published manuscript from one journal if accepted by another journal.