As long as the stage of the project is in Evaluating, you will be able to edit and change your scores as many times as needed.
You can see and change any Scores pertaining to a Vendor's submission by going into their Scorecard. Simply click on the Submissions heading, find the vendor you'd like to change your score for, and click the Scorecard button.
Once you are in the Scorecard view, you can then click the Edit button for the criteria you'd like to change your score for:
After clicking the edit button you will be able to change your score, your reason for the score, and any comments. Click Save Score to keep those changes.