To put it simply: You can't. As a project administrator, you cannot make any changes here. The setting of the username is reserved for each docu tools user themselves. You can only ask the relevant users to change their name in the User settings. You can refer them to the "Personal Settings" section in the article User settings. However, if you have rights to License management in your Organization, you can change the names for internal users (i.e., members of your own Organization). But you do not have access to external users even with License management.
The exception is Contacts. Since they do not have their own docu tools Account, you can Edit the Contact information at any time. To do this, go to the Contacts tab in the project members view. Open the menu by Clicking on the ... next to the relevant Contact and select Edit. Change the data as desired and do not forget to confirm the changes with save.