A diagnostic exam is meant to give you a snapshot of how you're doing at the time you take the test. Once you've seen the questions, the diagnostic power of the exam is lost. This means a retake would be much less accurate. For this reason, there is no way to reset your diagnostic score or retake the exam.
That said, we are always interested in getting feedback from students. If you'd like to add your vote for an option to retake the diagnostic exam, we would love it if you reached out to us at firstname.lastname@example.org.