Depending on exactly which Ruida controller the machine has, the option may be under something like Menu → Screen Ref Set.
On my KT332N:
That option does not appear in the manual and I assume the same is true of all the other Ruida controllers, so poking around on the machine console will be the only way to find it.
That said, IMO changing the machine units is a terrible idea, because exactly nobody you ask for advice will expect the units for a CO₂ machine to be anything other than millimeters. Just look at the chaos with diode lasers where “6000” might mean either, but ranting that units matter got me slapped down hard.