The entered user credential must be a Global Administrator to add, remove or modify licenses. For more details about assigning roles,refer this Microsoft article
How can I view the license information about my Organization and Users?
Using License Reports section in the Office 365 Manager module, you can view all the information about the licenses owned by the Organization and the licenses assigned for each user.
What is the difference between Add and Replace options given in the License Changes step?
If Add option is selected, the selected licenses will be added to the existing licenses for the selected user(s). If Replace option is selected, the existing licenses will be removed and new licenses will be assigned for the selected user(s).
When you migrate to Office 365, there in one critical element in managing your Office 365 environment and that is your Office 365 License usage, because this is what burns your dollars based on the Office 365 license plan you have subscribed to. You need to get insights on how your Office 365 licenses are being put to use. Click here to know more.