To my knowledge you can't change the fltHeader Id. You can't change it with Creator's attribute window, and you can't change it with the OpenFlight API.
The reason the docs say "usually set to 'db'..." is because if other tools create the file, they may have named it something else and OpenFlight parsers you implement should not break because of it.
Can I ask why you want to change it?
Only to verify that I have 'touched' the file once it has been processed. Another way to verify things are OK with the files...
I would suggest adding a comment to the header comment that includes the date/time etc...
mgSetComment (db, "any string here");
The spec for OpenFlight states that bytes 8-15 are for an 'ASCII ID' that is usually set to 'db'.
How do I change this via the API?