Editing an appraisal is simple. Simply click on an appraisal in your list, and it will slide over to the details of that appraisal. Everything on here is editable (unless you have restricted permissions - if this is the case, contact the admin of your company) and is underlined in a blue dotted line.
Click on any field underlined in blue, and a pop-up will appear. Make your changes and click the green check mark. This applies to all fields on your appraisal!