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

  • Go to Settings, 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
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