Is it possible for a candidate who already submitted their response to change it later on?

Yes. The links in the invitation emails do not have an expiration date, so if a respondent wished to change their response they could always click on the link in the email and overwrite the previous response.

Please note: the only exception to this are surveys with a closing date. The closing date feature is available for Employee Engagement accounts. Check out our FAQ article on adding a closing date to your survey here.