From the Users page, you can take away a DID from its user simply by clicking on the DID number of the user. A window will open. Select None in the DID field and click Submit. You can also do the same operation from the DIDs page. For more, go to Unsubscribing DIDs from Users.
As the company administrator, you can also assign DIDs to existing users from the DIDs page. To learn more, go to Assigning DIDs to Existing Users.