To add an additional account, click on the button.
The login window will appear, and after successfully logging in, the active account will be switched to the new account.
To switch between logged in accounts:
Long-press/click on your avatar in the Flume main menu.
Click or long-press/click on your avatar on the upload window.
To remove an account, simply log out of the account.
To remove all accounts:
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.