What means "change username"?

I want to change my username Atomic2 to Atomic2.0 or ATOMIC2 but I can't do it. When I click "SUBMIT" button nothing happens. What I need to do to change my username? Sorry for bad English. thx.

See here: lichess.org/forum/lichess-feedback/changing-username-2#1

Apparently you can only change the case of your username: (from Atomic2 to ATOMIC2 for example)

Thank you very much

But when I click submit button nothing happens...