In the Utilities menu, you can access the Drawing Setup option. From there, click on the Units tab. The dialog box that opens has all the tools you need to change your units. You can change them from inches to mm and also the decimal placements and precision.

If you’re talking about changing the units for a dimension style, just click on the dimension you want to change and type in the DIMSTYLE command. You should be able to change the units from there.