Transferring user licenses

In order to be able to transfer licenses, please follow these steps:

1. Inactivate a user in DESelect Segment from the Admin Panel > Users, to ensure there is one available license to assign to the desired user.  
2. Make sure the new user you want to activate is enabled in SFMC > Set Up. 
3. Ask the new user to access DESelect Segment. Now the new user will appear as active in the Admin Panel > Users. 

Related Articles: 

Salesforce documentation on assigning users access to Installed Packages

How is the number of active users licenses used calculator? 

Enable DESelect Segment in new Business Units