Digital trimmer capacitor
There are other ways to frequency-modulate a signal than changing a capacitance, but that's one of them.  Look up something called a varactor diode.  This is a diode whos purpose is to be a voltage-controlled capacitor.  The diode is reverse biased.  As the reverse voltage accross it changes, the size of the depletion region changes, changing the capacitance across the junction.

To control it digitally, use one of the many ways of converting digital values to a analog voltage.  Low pass filtering the PWM out of a microcontroller is one simple way, assuming you already have a micro in the system.