Click or long-press/click on your avatar on the upload window.
Removing an Account
To remove an account, simply log out of the account.
Use the Log Out…menu bar item when logged in with the account you wish to remove. If you are logged in with multiple accounts, use the Log Out of @usernamemenu bar item instead.
To remove all accounts:
Use the Log Out of All Accounts…menu bar item. This menu item will only be visible if you are logged in with more than one account.