Can't be done.
Although we have a rule against multiple accounts for one user, for this circumstance you could always abandon this account and make a new one.
I recommend coming up with a shorter username as we have a rule against excessively long names.
Registration of excessively lengthy or trolling nicknames are also prohibited; this includes names composed of many all-capital letters, and/or mostly non-alphanumeric characters.
Please read the forum rules before making a new account