Click on Pieces in the left-side menu. Click on the Piece with the Appraisal you want to edit. 

Go to Appraisal History on the right and click on the Appraisal you wish to edit. 

Then click Edit Appraisal.

Update the Appraisal and click Save. 

NOTE: Appraisal Tracking is only available on Collector and Organization plans.