User identity in Flow is managed by the underlying service (like Salesforce, Box, or Google). If an app user has forgotten her password, a user reset should be performed on the underlying system.
If you are looking to reset the password of a Flow builder, please see here.
If you have forgotten your Boomi Flow username, please get in touch with us to request a reset.
If you do not have a Boomi Flow login, click here to get one.