Hi Kumar,
You'll need to write to the appropriate register for the set point. Based on DDang's code and the programming manual, it looks like Modbus register 300. I didn't browse the entire manual, but once you open a Modbus serial connection to the controller you should be able to write to the register at your desired rate. DDang's code is a good starting point keeping in mind RavensFan's last suggestion that the initialization and closing of the serial port should be done outside the while loop.
If you have any additional specific questions, I would recommend creating a new thread so your post doesn't get lost in older threads.
Best Regards,