Default Display Name

“Display name publicly as” is the field you are looking for. You can see it on the screenshot of this Codex documentation page:
https://codex.www.ads-software.com/Users_Your_Profile_Screen

Using this approach, you would have to go into each User’s profile and make that change. I’m not aware of any easy ways to make that change for all Users in “one click”. There may be a plugin for that, but I suspect, if I had to do it for a large number of Users, I’d be putting on my programmer hat and writing a database query to make that change directly to the WordPress database. Ugh!

Thanks, I know how to do it manually. I have already changed all the users manually, but would like to avoid it for future registrations.

Thanks!

OK, that is a different question. Assuming that you are asking how to have all future users display as Username instead of Nickname. I’m not aware of any way to do that within WordPress itself without do a lot of programming.

There are plugins that can help. I stopped looking after finding a rarely used one that would seem to solve the problem: https://www.ads-software.com/plugins/display-author-option/

One note of warning, by displaying the ‘username’, you expose your site to breakins. Since the username is also used for logging in, hackers just need to guess the password to gain entry.

In stead of displaying the username, consider editing and using the nickname with something more acceptable.