Your Display Name (first name, last name) in formed by concatenating your first name and last name as specified in "My Geek, Profile" by editing "User Details" at the bottom of your Profile. (Click on the pencil icon to edit the "User Details" section.)
You can change your display name at will, although it will be confusing to other users if you flip you display name and/or avatar frequently.
See User Profile for more details on the "User Details" of your profile.
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