If you have changed your mind about your experience with a Seller 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 Seller. Feedback is only left after you have paid your Seller'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.