A common change that needs to be done to the units of a *.c2 file is to convert it to use OD units for diameter, rather than nominal units.  When designing systems with tubing, the OD is a required variable. By default, the units of CAESAR II are set as Nominals active.  Here are the steps to turn nominal units off. 

Go to create/review units
Follow these clicks to make a new units file, base it off of whichever existing unit file you want.
When you edit the new unit file, make sure you set NOMINALS = OFF
Click OK /Save, then review the new units file you created, to verify you set nominals as off.
Then go to "change model units".  Follow the dialog prompts to convert your existing file to use the new units file without nominal sizes active.
After that, open up the file you converted.  You should be able to type in exact values for pipe OD now.