In order to re-use a set of credits for a participant taking the place of another participant, refer to the How to Remove Participants From a Project tutorial, first.
Once you have removed the existing participant, you will regain their credits to use for the new participant taking their place. Refer to the How to Add or Edit Users tutorial for instructions on adding the new individual, if necessary.
How do I remove a user who will not be able to attend class?
How do I transfer an unused license?