Never have been able to find the option anywhere, so is this not possible or am I just blind?

Sometimes I check out a game and want to review it, but feel like I should hold off just in case the author addresses some of the issues it had. (Yonder being the most dramatic recent example.) I keep thinking it would be REALLY helpful to be able to revise a review and increase the rating if someone posts a game with problems but then later improves it.

Seconded. I almost NEVER rate for that reason. I comment the sh!t out of some games but rarely rate.

Good point. This seems like something that should be encouraged, allowing authors to improve their games in response to feedback. I will see what I can do!