Whenever you decide you need more user seats in your organization, you can add or remove them as you wish. When you add seats, the change is effective immediately and your card will be charged with the cost of additional seats.
When removing seats, first make sure to delete users that go over how many seats you want to have. Once they are removed and the seats are free, you can change to a lower seat number in your plan. Lowering the number of seats happens immediately. To change your seat number, go to the Administration panel and in the Billing section, click on Plan. Click on the ellipsis on the right side of the screen and select Change seat number.