How I can change the Username itself, not just the password.

There is a security risk in not allowing the name of the admin user to be changed - this is what's called two factor authentication, you need to know two things to get access, the ID & the password, and you already know one. A few decades back, when I was doing IBM midrange support one of the presenters at a seminar was touting the security of the AS/400 series as unparalleled until I pointed out exactly what you see above, OS400 would actually tell you whether the username or the password was wrong - contrast that to many platforms that will only tell you that one of the two is wrong, leaving you to figure out which of the two it is. Back then, I could spend half an hour chatting with a machine operator in the break room over lunch, and then walk up to their workstations and login as them, typically guessing their password within three tries - IBM training provided specific examples to passwords, and many of the users, not realizing that those were simply guidelines, took the examples back and used them literally - husband's names, children's names, that sort of stuff. Not allowing the username to be changed is a support issue, we are dealing with consumer product that typically ends up in the hands of users who, given the ability to change more that necessary, will get themselves deeper into trouble.