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 a role that is allowed to invite users

  • 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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