It's not always possible to change the processor core voltage through software. In fact unless the board is designed specifically to allow the VRMs to be programmed through software it isn't. Normally the VRMs are programmed through the Vid, or voltage ID, pins on the processor and set the voltage based on those pins. The cpu has no control over that and thus there is no way to alter it through software. In order for it to be programmed through software the VRMs and the motherboard must be designed specifically to allow this. Motherboards that allow you to adjust voltage in the bios are designed to allow this. So, if your board doesn't allow voltage adjustments in the bios it's likely because it's not possible on that particular board.
That means that in order to alter processor core voltage you'll have to do something with the Vid pins. It's always possible to alter voltage through the Vid pins (to a certain extent) as this is Intels standard way for a motherboard to determine what core voltage a CPU requires. Thus when you alter the state of the Vid pins you are tricking the motherboard into thinking this is the defualt voltage the CPU is suppose to run at. Of course even altering the Vid pins has it's limits. The VRMs will only be capable of supplying voltages within a certain range and if the Vid pins are altered to request a voltage that the VRMs aren't capable of supplying then they will supply no voltage whatsoever and the system won't power up. That's a safety feature. An example of this would be if you had an old slot 1 board that couldn't supply voltages any lower than 2.0v and a 1.5v cpu was inserted then the VRMs would see by reading the VID pins it was a 1.5v CPU and required a voltage that was out of their range of being able to produce and never supply any power to it. Thus the CPU wouldn't be destroyed.