There are two primary units systems available for selection in STAR-CCM+:
To set the SI or USCS systems as your preferred system, simply select it in the Units node in the Tools node:
What happens when you change the system of units is that STAR-CCM+ runs through all the units and sets the preferred ones to be the SI or USCS selection.
Keep in mind that any time a new preferred unit is selected it will reset the units everywhere in the system to that new value. Thus, it will reset any manual overrides you may have set in specific quantities.