If you're trying to edit the Periodic Review of a Published Document 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?
Relevant for: Administrators / Managers.