An already completed evaluation can be reopend by either a manager or an admin. You can do this by going to the evaluation form that you want to reopen.
Note: If you open an evaluation again, it needs to be completed again by the manager (and possibly approved).
When you navigated to the correct evaluation form, click on the three dots and then on 'Edit evaluation status'.
In the modal, choose 'Re-open' as the new status, and click on Edit status to re-open the evaluation.