You may realize you made a mistake only after you've submitted your feedback, or you may just want to elaborate on some of your answers. This is not a problem, you can easily reopen the feedback form to make modifications, as long as the deadline of the corresponding survey has not passed. If the deadline has passed, a manager or administrator will need to Extend the deadline of the survey.
To modify the feedback you've submitted, find the self evaluation in the Completed Tasks section of your To Do list and use the "Reopen" button to access the evaluation form and make your modifications.
Once you're done with the modifications, click "Submit" to send your updated responses.