The reference library does not indicate any way to control the PWM frequency via Arduino code. It appears to be fixed at 490 Hz (980 Hz for pins 5 and 6 on the UNO). I only see a way to adjust the duty factor with the analogWrite(x,n) function. See analogWrite() - Arduino Reference for details.
Could you accomplish your goal using the tone function? It allows you to change frequency?
(Duty factor fixed at 50%)
See : tone() - Arduino Reference