You can only change the serial port baud rate of the PLC by executing the SETBAUD command INSIDE the PLC program
E.g. use an input to trigger a custom function and inside the custom function you enter the SETBAUD command. Once the input is triggered and the function is run the baud rate will change.
SETBAUD command is NOT to be entered from the TLServer (I assume that is what you mean although you said "Fserver" in your quesiton). If after the PLC executed the SETBAUD command and you want to use TLServer to talk to the PLC using the new baud rate, then you need to change the baud rate, data bit, stop bit etc on the TLServer serial port setup.