As an end Customer you cannot amend your own licences via the portal.
You will need to contact your supplier to request these as they are added at the Account Management level.
You can see how many licences you have available and whether any of your users are unlicensed, from the Users tab in your portal. Failing that, you would need to refer to your supplier.
Microsoft Direct Customers
You will need admin access to your Office 365 account.
- Access the Office 365 admin center at https://admin.microsoft.com/
- Ensure you are using the latest Admin Centre.
- Go to Billing menu on the left, select Your Products
Scroll down and find the relevant service in the Apps section.
You will be able to then Add/remove licences using the 'Buy Licences' or 'Remove Licences' optoion.
A licence increase/decrease should be effective immediately