To manage license assignment in the Classic (Aloha) experience, navigate to Setup > Installed Packages
and click "Manage Licenses" next to the app you purchased. To manage license assignment in the Lightning Experience (LEX), click on the "Setup" icon in the type right, and then type "Installed Packages" in the Quick Find box on the left side. You can also navigate to Platform Tools > Apps > Installed Packages
from the Setup screen.
Search for, select and click "Add" for each user you want to assign a license to, up to the license quantity you purchased. If you have a large quantity of users, you may want to create list views (based on profile or role, for example) and use the "Add All Users" button.
In the event you would like to remove a license from a user (such as when a user switches roles or leaves the organization), simply click the Remove button next to the user's name under the Licensed Users section. If you have a large quantity of users that you want to remove licenses for, we recommend utilizing the Remove Multiple Users button.