How to change a predefined PWM frequency?
There doesn't appear to be a "quick and easy" way.

A quick look around the ftcommunity project and related forums says that the low PWM frequency is often a problem.  

The most common solution seems to be to attach an external PWM generator to the TX controller.

Some of the posts mention using the Adafruit 1438.

The newer versions are I2C, and have a built in PWM generator.  Frequency and duty cycle controlled through I2C.

The TX seems to have an I2C port and software (judging by the forum comments.)