To edit or delete a Reviewer's score as the Project Owner, navigate to the Reviewer Scores section and expand it and select the reviewer. In the example below we have Ada's scores in context.
Click on the criteria you want to edit or delete the score for. With the criteria open, click the Cancel button:
Hover your mouse pointer over the row of the criteria and you'll see both an Edit button and a trashcan/delete icon.
Reviewers will be notified of any changes made to their score.