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You can manually change any set up already from their profile.

For new users you can change this at registration
eg

function change_display_name( $user_id ) {
 $info = get_userdata( $user_id );
 $args = array(
 'ID' => $user_id,
 'display_name' => $info->first_name . ' ' . $info->last_name
 );
 wp_update_user( $args );
}
add_action('user_register','change_display_name');

you have options of
‘display_name’ => $info->user_login
or
‘display_name’ => $info->user_email
or
‘display_name’ => $info->user_nicename

or if you have a custom field, you can use that

Put this in your child theme’s function file –

ie wp-content/themes/%your-theme-name%/functions.php

where %your-theme-name% is the name of your theme

or use

Code Snippets