It's the one highlighted that I wanna change but it won't lemme ;u;
That's your
Forum display name, you can't change your
username (to my knowledge) once your account has been made. You can however contact an Administrator, to perhaps delete your account so you may create a new one under the username you desire, however it's an unlikely possibility that it'll happen. You'll have to contact one of them, so you can discuss it further with them. If you like, you can change your display name, so it's your in-game username, and that's how your name will appear to others. But your name via the forum to log in, will not change.
An example is:
I can change my
Display Name to be "Honey Bee-Bee" and this name, is how my profile would appear to others, as a
Display.
But, my
UserName is still "Papercut" And it cannot be changed, I'll still need to use my
UserName to log into the forums, not my
Display Name.