It is not possible to edit a completed evaluation; however, the evaluation can be purged and scores can be reassigned.
Open the Evaluation records tab, find the evaluation you wish to omit from the scores by filtering on completed evaluations. Next, click the View Evaluation Report icon, then scroll to the bottom. Final step is to click on the Purge and Resend button to send a fresh evaluation and clear all the old data for that particular evaluation. This option maintains the rest of the evaluation preset.