The manager's assessment is automatically assigned to the employee's default manager listed on their profile. Admins can change this reviewing manager once an employee has started their self-assessment (as this is when the review is created in the database).
Go to Reviews overview > All colleagues and find your employee's self-assessment in the 'In Progress' tab. You can use the Filter by team member search to find them quickly. Click the pencil icon next to the Manager field to update the reviewing manager.
You'll be taken to a new window where you can select the new reviewing manager from the drop-down list. Click Update review to save.
The new reviewing manager will now see the employee's review in the Direct reports area on their Dashboard, and under Reviews > Direct reports, where they can click to provide their assessment.
This process will not update the employee's system manager. It's a one-off task to change the reviewing manager just for this review.