How do you increase the speed limiter to a 2008 Volvo fh 12?

You need Volvo's software to do this, and it's not cheap, whatsoever. You also need some means of accessing the electronic engine controls, such as a control box, or laptop computer with compatible connections.

If it's limited, it most likely either isn't your truck to make those changes to, or else is governed because your local/national laws dictate it, and you'd probably be best to leave it alone. But you can still envy me for driving an ungoverned truck.