My apologies as I was just made aware that we are not receiving emails on the forum questions. I am sure that you have already done with this but for future reference, please see the response.
Are you referring to the Student Portal? You would have to make the test available again to the whole class, delete that student's scan so that when the student logs in, the assignment is available to take again but the whole test would have to be taken.
An easier way if for you to just go in and edit the student's answer in the scan area and update there.
Again, I apologize for not getting right back on this.
Please let me know if you have any other questions.