Can I decide myself what to review and how often I review?

Yes, this is possible on Log in on a computer and click on Vocabulary at the top of your Babbel homepage to open your vocabulary list. The Filter option at top left of the list allows you to sort according to Theme or Knowledge Level. You will then have the option to Review these items.
You are also able to manage the frequency of review by moving words or phrases to different knowledge levels. To do this, simply click on the knowledge level you would like to assign to each vocabulary item.