How to send commands to iRobot Create 2?

There is documentation on the website about this very robot.

https://www.irobot.com/about-irobot/stem/create-2/projects

I suggest you go to the bottom of the page and look at the Arduino and Raspi pdf files that they provide. The Raspi documentation tells you how to command the robot. I think from there you can follow their instructions. I would provide more of an answer, but it seems the OP simply does not know where the docs are for their robot.