Your Billing admin can add or remove licences in the Billing portal. Here's how.

1. Click Settings → Account → Billing portal:
   
   
   
2. Next to Used CRM licenses, click Edit seats. For support licences, click Edit support seats next to Used support licenses instead (see How do I add more support licenses? and Licence types):
   
   
   
3. Click + or - to change the number of seats. The rate changes. If your plan has add-ons, click Show pricing change for a detailed breakdown:
   
   
   
4. Check I am authorized to complete the purchase and click Add or Remove (for example Add 2 licenses) to confirm changes:
   
   

Added licences can be used right away.

Removed licences stay on your account until the date shown in the drawer, the start of your next renewal term. You have already paid for them for the current period. You can only remove licences until 3 months before your renewal date. After that, the - button is greyed out until the next term starts. You can't remove licences if your subscription is cancelled.

If your account has a special discount, changes to your agreement are made through your account manager.