I don't know about how to program the PWM to use different frequencies (that's what I was trying to use the forums to find), but mathematically speaking there's an upper limit to how fast the board can output a signal. The clock operates at 16 MHz, or 1.6*10^10 Hz. The PWM has a minimum resolution of 8 bits, which requires an 8-bit timer. If the timer counted the clock directly the highest frequency the board could output would be (in theory) 1.6*10^10 / 2^8, or 62.5 kHz. If you can configure the timer to be 8 bits instead of 16, you can use prescalers and gain values in the same area of the code to drop the frequency from there.