This works only when a user has UDATE right on users. a guest profile cannot update himself from the API currently (see https://github.com/glpi-project/glpi/issues/2568)

The agent may update user informations like phone number, emails, name.

This can be done with the endpoint User. Only fields requiring an update should be specified.

PUT apirest.php/User/:id

{
    "input": {
        "firstname": "new firstname",
        "realname": "new realname",
        "phone": "0123456789",
        "phone2": "0987654321",
        "_useremails": { 
            "0": "[email protected]",
            "1": "[email protected]"
        },
        "_default_email": "1",
    }
}