System generated username

How can I change the system generated annoying username? In the account preference settings there is no option to do that.
Only thing I can add is my real name, but is optional.

So why is such an obvious function not available?

Your forum name is tied to your Xbox Gamer Tag, once you change your Gamer Tag log out and back into our forums and the change should take effect.

Please visit the Xbox Support Website for information on changing your gamer tag.

Hi Nyx1819, thanks for the swift response and the helpful solution.
Cheers, Strobeoff