If you have changed your mind about your experience with a freelancer after you have left feedback, unfortunately, it can't be changed. You should leave honest feedback that is a true reflection of your experience with the freelancer. Feedback is only left after you have paid your freelancer's invoice for work completed, and you should only pay invoices when you have checked the work delivered.
However, if you made a genuine typing error (for example accidentally selected 2 stars when you meant 5!) then contact PPH Customer Support immediately.