What do you mean with units?
- Units on parameters, you need to declare them at creation in C TK. Unitssytem of files ?
- You need to change them on every part / assembly and on your Start Objects (parts, assemblys) to get it permanent same. You can do it manual, by mapkey, or by an external Tool like Model Process (http://mp.inneo.com, free for single parts after registration) or SmartUpdate (B&W Software GmbH, I think you can get also test license, ...).
From the above mentioned path, select the Detail Options link and it opens a window for you to be able to change parameters as you see fit. You can even save your changes to an external file and then import it later on as needed. Altho I think that the changes you make in the Detail Options page is only applied to the active drawing.
If you are talking about setting the units through toolkit each time you open your session you can use ProMdlPrincipalunitsystemSet() and call it in your initialize module.
manually you can change the same in ModelProperties under Prepare.