Long story short: You can't. You as project administratork cannot make any changes here. The setting of the user name is reserved for each docu tools user. You can only ask the user to change their name in the user settings. You can refer them to the "Personal settings" section in the user-settings article. However, if you have licence management rights 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 are contacts. Since they do not have their own docu tools account, you can edit their contact information here at any time. To do this, go to the Contacts tab in the project member view. Open the menu by clicking on the ... for the contact in question and select Edit. Change the data as desired and do not forget to confirm the changes with save.