Sign in to Office 365 using an administrator account.
Then, chooseAdmin.
Go to Billing > Subscriptions
On the Subscriptions page, select the subscription that you want to add licenses to and then select Add/Remove users.
In the Total licenses box, enter the total number of licenses that you need for this subscription and then select Submit. For example, if you had 100 licenses and you need to add 5 more, you would enter 105.