If you have already submitted your assessment and it is not yet closed, you can still edit it.
To do so, navigate to the 'Reviews' tab > Dashboard > Ongoing assessments. Here, you'll find the reviews you've completed of yourself and other people. Expand the assessment of the user whose assessment you would like to edit > click on the specific task that you completed to open the assessment.
When you've reached the page shown in the screenshot below, click on the 'Edit' button in the bottom right-hand corner.
Please note: editing is no longer possible after the review cycle has closed.