The availability of the U axis for rotary use is dependent on your laser’s controller and the firmware version it’s running. If your laser supports using the U-axis for rotary, you keep it set to Y in LightBurn (that should be the only selectable option for most Ruida controllers), and the controller handles transferring the commands to the U axis.
What specific model is your controller, and what firmware is it running? You can check the firmware version by going to Edit → Get Controller Info.