I was able to resolve the issue , the primary error I was looking at the worng code to change. I assumed that changing baud rate in serial_bridge.py in rosbot_description package will change the baud rate for rosbot 2.0 pro.
I followed tutorial no - 3 from the rosbot tutorials and the rosbot started moving. The baud rate needs to be changed in the microcobtroller to establish connection between ros running on the upboard and microcontroller running the motors.
I have question what microcontroller was used and how did you get microcontroller and ros to work together. Is there any offline software to write and compile these programs directly on upboard. or do it in a offline computer/desktop and burn it to rosbot using a cable ?