You can't change selcted username, but you can create new one. Usernames are forever, to prevent identity conflicts, (if people would use username that has been used before).

This is how to do it:

Log On with your present username, and choose to edit your email address, change it into any legal email address, like for eample: em@em.com

Then Log off.

Now, you can use your real email address to create new username.

Choose link "Join" at the top of every page, and use your real email address to create new username ...

Now, you have 2 different usernames.