You'll see many other options there too, like; Full Name, Gender, Date of Birth, Country, City, Username, Email, Password and Mobile Number, and all of them can be changed / updated many times, other then the Username!
Username can only be changed / updated for once, and can only be changed / updated from here!
Once set, it cannot be changed / updated!
Your ID / Username is your identity and on the Forums; members can tag you by your username, so choose it wisely and carefully!
(Avoid choosing inappropriate or offensive usernames, like *orn star, hell raiser, etc. and also do not set your contact no. as your username. It may be a symbol of immaturity, and you may never know, who your audience is!)