I want to change my username

The only way is to de-activate your profile and start again.

You'll have to deactivate your current account and create a new one.

-- !!!CAVEAT EMPTOR!!! --
IF YOU DO DECIDE TO GO THIS ROUTE, KEEP IN MIND THAT YOU MAY NOT USE THE SAME EMAIL ADDRESS FOR YOUR NEW ACCOUNT! YOU WILL ALSO HAVE TO PROVIDE A COMPLETELY NEW EMAIL ADDRESS IN ORDER TO CREATE YOUR NEW ACCOUNT!

Mingle2 only allows one account for any given email address. If you use your current email, create your account and goof up on your user name, you will have to make an entirely new email address as well as your new account. NO USER SHALL HAVE MORE THAN ONE ACCOUNT PER EMAIL ADDRESS. Be original, think twice and name yourself only once so you don't later have to generate a new account.