The short answer is, you can't.
However, you can use this workaround to get the username you want.
Making your current email address available
Use the Profile menu in the top right
Change your email address to something else, so it's available to use in the next step.
Creating a new user with the right username
Next, go to Settings in the bottom left corner of your screen
Next, click Users
Click the Invite users button
Enter your original email address and pick a role that is allowed to invite users
Accepting the invite and archiving the old user
In your mailbox, click the Invite link
Sign up with your desired username and password
Go to Settings, next click Users
Find your old user and click the Actions button to Archive it