How can I withdraw a manuscript that is placed under-review? I have a manuscript that is placed under review for a month now and during this month I have discovered that there are a few things I want to change/add in the manuscript and I don't want to publish this manuscript in this particular journal anymore. 

How can I ask for this manuscript back? 

Should I just send an email to the editorial office and what do I say? 

This is the first time I done such a thing.

Thanks,
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.