Thank you for the answer! I have already changed the option from there. However there is this message: Warning: If you change the display name format, it may take some time on a large number of users to update them to the new format. Please be patient.
I quess the new users will be displayed with first and last name. However how can force the existing users to have the same settings.
Furthermore when I am editing users permissions settings. There are some users with their emails, some are with display names, very confusing for me. I want to rewrite the database or change settings that it is a must every where to view First and Lastname – in our private forum and admin settings.