To change the due date of a single employee's performance review you should follow the steps below:
Click on Manage Reviews on the left hand menu
Click on the burger menu of the template you are looking to extend for an employee and select Periods
Look for the review period that is current and fully expand to see the details of who is in the review
Search for the employee that is being evaluated by the employee needing more time. (For example, if Jim needs more time to complete Jennifer's review you would search for Jennifer in the search bar)
Click on the burger menu to the far right of their name
Select Edit Period from the menu that is shown
Change the dates to be the new range you wish for them to be able to complete the review
You can also watch the video below to see steps on how to complete this task: