You can increase or decrease the number of agent licenses you have at any time. All license changes can be made from the Billing Dashboard within your account:

Increasing Agent Licenses: 
Increases are charged on a pro-rata basis, so to increase the number of agents, you simply pay a time-adjusted fee - the annual rate for the extra agents, multiplied by the fraction of the year or month that remains in your current billing period.

For example, to add 10 agents, 6 months through an annual billing period, you'd need to pay half the annual rate for 10 agents.

Cloud Monthly: 
Adding new agents in the agent area automatically increases the licenses in the billing area. The pro-rata charge for the increase will be taken automatically on your next bill date.

Cloud & On-Premise Annual: 
You can adjust your agent licenses by going to the Billing Dashboard within the helpdesk. You'll be required to pay the pro-rata charge upfront (via card or BACS/wire transfer) before additional agents can be added.

Reducing Agent Licenses:
You must reduce your agent licenses on the Billing Dashboard within your account. You need to ensure that the number of agents in your helpdesk is equal to or less than the number of licenses you wish to decrease too, so you may need to delete agents before you can reduce your agent licenses. Deleting agents does not automatically reduce your licenses or billing.

If you reduce your agent licenses, you will not be refunded pro-rata and will be billed your new price at the next billing cycle.