If a feedback-giver would like to edit the feedback they've submitted on a colleague's review, they will need to be re-invited as a feedback-giver on the review.
Whoever is allowed to invite feedback-givers on your account - reviewees, reviewers and/or administrators - can also re-invite people to make edits.
Follow the regular steps for sending feedback requests.
By re-inviting you, their response will be put in draft mode and they can edit this from their Feedback tab. Once they are happy with their amendments, they'll need to submit their feedback again.
Note: re-inviting a feedback giver will not remove the submitted response from a review - this is only updated once they re-submit.