If you're trying to edit the Periodic Review of a Document that is not yet Published status, in the Document settings but you are unable to do so because the Periodic Review dropdown is greyed out, it might be that your role within the Document - i.e. Author, Reviewer, or Approver - is not allowed to edit the Periodic Review.
To update the roles that are allowed to edit the Periodic Review, please refer to the linked article: How do I define who can update the Periodic Review Date?
If you are trying to edit the Periodic Review of a document that is in Published status, then from the Document viewer page, you must select the Calendar button option, and select a new Periodic Review from the pop up that appears.
Please note: Once a Document is in Published status, the option to edit Periodic Review from the Document settings will not be possible.
Relevant for: Administrators / Managers.