I enable the option, only registered user can comment on an article. But, now the Name field of the comment form is autofilled by real name. It seems some file overrides the file, components\com_k2\templates\default\item_comments_form.php. But I couldn't find it. Please help, thank you.

Hello,

this was explained in an older post which is not acessable any more :(

To have usernames instead of real names you need to modify the files in the installation package,
search there for "user->name" and replace it with "user->username".
I use for this the program Actual Search and Replace.
Maybe you need also to modify the same for "author->name" to "author->username".

I have done this for my website and will share it with you:
K2_v2.5.6.-username.zip

Hope it will help.
Because if you are waiting for any reply of the coder here, you will be in your future life :(

Cheers,
SolarShip

AHH, the attachment will not work :(
look for other solution.

HERE WE GO: se.ma/2-256-username