VCO : voltage controlled oscillator
is a kind of oscillator, that is enable to change its frequency
due to some external voltage applied across it.
A probably most important component is a varactor diode.
You can refer a book about varactor diode. If you apply
a varying voltage across it, the capacitance of the diode
will change in it rhytmus, so with a fixed inductor you can alter
the frequency.
To demodulate an FM signal, you can use a frequency discriminator,
simply saying, you need a filter with linear flank.
regards