Updated Beliefs is a report of general beliefs or perspectives that one has substantially changed their mind about. A post on this may share what one used to think, what the new views are, and what caused the change. Sharing such accounts publicly is prosocial and allows others to learn from one's experience too.
This tag is specifically for changes in general beliefs about the world – what you believed before, what you believe now, and causes of the change. If a post focuses reporting actions and outcomes together with an evaluation of the choices/thinking patterns used, then it is a good fit for Postmortems & Retrospectives. Central examples of Updated Beliefs posts are “'Other people are wrong' vs 'I am right'” and “Six Economics Misconceptions”; in contrast, a central example of a Postmortems & Retrospectives post is “Arbital Postmortem”.