Login to the portal site and navigate to the studio section (usually the url will be in the form studio.<regular portal name>/home).
To navigate to the studio from the original site, you can also go to <site>/admin to get to the admin settings and go into the courses drop down on the left navigation bar, shown below.
You will see this type of template when you get to the studio page.
Find the course that the student is enrolled in and unable to retake the exam for and go into it and click on View Live in the top right corner.
Once in view live, move into the Instructor tab and from the instructor dashboard go into the special exams tab, search for the student unable to retake their exam and then from the actions column click on the "[x]" button. There will be a confirmation message asking if you would like to remove their attempt, press "Ok" and the student should be able to reattempt their exam once again.