You can reset passwords, set licenses and roles, disable or enable Office 365 sign-in, add new aliases and change which email address is primary, and perform many other functions, via the user panel.
To access the user panel, simply click on an active user, and it'll open to the right.
A lot of these functions are very similar to the equivalent that you'd perform for new users. For example, resetting a password is just like setting it for the first time for a new user:
The user panel
Assigning a license is just like assigning a license to a new user. But there are some functions that can be performed via the user panel ...
