account names cannot be changed.

you can only change your display name, which is available in the settings.

whether display names or usernames are shown depends on the interface/client and user settings where available.

the only way to change the username is to create a new account.

The display name or the actual account name?

I've not heard of this setting so idk. But to not-answer your question, I just make a new account. It doesn't take long before you're subbed to all the same things, etc