In the options for the drawing,
File->Prepare->Drawing Properties
select "change" on the "Detail Options" line
Scroll down to "dual dimensioning"
if you're currently using dual dimensions this is set to "primary [secondary]" or "secondary [primary]". You will probably have to switch this from one to the other of these settings to make inches or millimeters the non-bracketed unit.
If both are being shown in the same units, you will need to scroll down to "dual secondary units" and change it to the unit of measure that isn't used by the model (i.e. if the model is in inches, set this to "mm").
