How to set USB CDC bitrate on STM32?It seams that there is no interface like function or register to operate,so is it editable?
You don't set bit rate yourself on STM32 firmware, unless you're writing your own USB CDC stack. CDC driver on host (PC) makes control transfers to set the bit rate. These are generally triggered by the application on the host side, like a serial terminal application.
If your STM32 application consumes/produces the serial data itself and doesn't act like a USB - Serial converter using physical USART lines, then the bit rate is irrelevant for the application.