If a student has submitted the incorrect paper to an assignment, and they are unable to resubmit the correct one, the class instructor can remove the incorrect paper from the assignment inbox and submit the correct one on behalf of the student or, if late submissions are allowed, let the student submit again.
Alternatively, the class instructor may update the assignment settings to allow resubmissions; this way, the student will be able to resubmit the correct paper and overwrite the previously submitted incorrect one as long as the assignment's due date and time have not yet passed.
If the student is trying to resubmit a paper after the assignment due date has passed, they will not be able to. Please note that this is intended behavior; resubmissions are not allowed once the assignment's due date and time have passed, even if the assignment allows for late submissions to be made. Late submissions are only available for first submissions.