Hello Zj.1,
Actually the HPEV board uses already UART as communication interface to any host system. The HPEV board only has a USB to UART converter IC to make usage with today PCs easier as most of them to not have physical UART anymore. Easiest way to use the HPEV board via UART is to simple bypass the FT231X IC. But be aware the the USB socket is also used to supply the board.
Regarding your second question to reduce tthe BR you have to rebuild the FW and your host application with your BR settings.
For the FW simple set the compile preprocessor define FAST_UART to 0, this sets the BR to 115200 in the FW.
For the host application you need to rebuild the STUHFL library after editing the file stuhfl_dl.c.
In this file change the following variable to your BR: static STUHFL_T_ParamTypeConnectionBR br = 1152000;
BR