by ben reynolds.
This is one of the reasons with give teachers to capability to "Login as." It can be done at the category or course level by an admin.
Once you have login as, you login as the student, go to where the file is wrongly located, download it, delete the file in the wrong place, go to the right place and upload.
Or, tell the students to do that work. Most of our 13 year olds have no trouble correcting their errors, as long as the submissions are not locked.