You will have to make sure that the number of seats you want to reduce are not being used by Agents. To make sure that you only have that many agents in your account, move to Admin > Agents. You can delete the agents but make sure that the tickets under the agents name have been assigned to someone else so that all the tickets are not unassigned once the agent is deleted.
Make use of the 'Bulk action' feature in the Tickets list view to reassign tickets to a different agent. Once all of this is done an account admin will be able to decrease the number of agent seats from Admin > Plans and Billings.