As long as a voucher is still valid and has time remaining before expiration you may reclaim then and reassign them to different students.
Go to the course:
Then the class you want to reclaim a voucher from. you can click the class name or "View Class"
Reclaim a voucher by pressing the "Reclaim Voucher" button:
You will be asked to confirm the action:
The voucher will then be ready to reallocate to another student.

If you are not the owner of the voucher (the teacher who allocated it to the student), you will not be able to complete this step, please ask them to reclaim the voucher: