An employee can save a draft of their self-assessment and return to the draft at any time under Reviews > Your review. Once it's submitted to the manager no further edits can be made.
If you've submitted your review and you want to make changes, get in touch with your HR team / account admins and they can set the review status back so long as your manager has not yet started drafting their manager's response.
If you're an administrator, our Help Center has a step by step guide to changing a review status at any point in the review process.
Customer Success Manager