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.
- Click Update
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 the Administrator role
- Click Invite
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