As a newbie to all of this, I realize when looking at other reviews that I was unfair in my rating for someone I saw.  Is there any way I can increase an already-approved review by at least a notch on each rating?

see her again and write a new review.  It will replace your old review.

To do that is to see her again and write a new review, dont think you can change the rating on an existing review. But remember your review is about your experience the time you saw her, not about anyone elses experience. honesty in reviews is what makes TER useful not just writing what others have said for better or worse.