Reassigning a seat to a new trainee is simple (and free!):
In your Dashboard, locate the trainee whose seat you would like to free up. (Note: when you free up someone's seat, that trainee will be archived to free up their spot. You can still see all your archived trainees in the Archived members section at the bottom of your Manage members page.)
Click Reassign next to their name.
Confirm in the next step.
The trainee's access to the Premium account will be terminated, and the license (seat) will become available to invite new members. You may want to let your trainees know about ending their access to avoid misunderstanding.