This can be done in 2 ways. Through the MagiCAD Settings or Revit's own Manage tab.
Go to the "MagiCAD Common"-tab => Settings => The "Units"-tab => "Change"-button to the lower right and then on "Discipline" you select the correct one, for example "Piping".
Then you can click on the different measurements and the different units for that discipline as well as edit the number of decimals used.
Go to The "Manage"-tab and then select "Project Units", then select the discipline you want to edit.
Supported units for MagiCAD are:
HVAC Air flow - l/s, m3/h, m3/s
HVAC Pressure - Pa, kPa, bar
Piping Flow - l/s, l/min, m3/h, m3/s
Piping Pressure - Pa, kPa, bar
Piping Power (HVAC) - W, kW