Its automatic, the operating system uses the EIST driver to tell the CPU what frequency to run at.
You can set it manually but there is normally no advantage in doing so.
In Windows to lock the CPU frequency at one value you can either: A. Change the "Minimum Processor State" and "Maximum Processor State" in to the SAME value in Control Panel>Power Options>Change Plan Settings>Change Advanced Power Settings. B. Or use a third party application like ThrottleStop to lock the CPU multiplier at any value you want.